Automatic self-test system for a digital multiplexed telecommunication system

ABSTRACT

An automatic self-test apparatus for use in a digital multiplexed communication system and self-testing idle circuitry coupled to idle telephone lines. The apparatus utilizes a tone generator which generates digital words describing an analog test signal and a detector which detects digital data on the internal data bus which describes this self-test analog signal. The data from the tone generator is coupled to the circuitry connected to the idle line through a time division multiplexer under the control of a controller consisting of a programmed microprocessor. The microprocessor controls when the transfer is made and to which telephone line it is directed. This control is accomplished by writing a control word to a multiplexer naming the source and destination for the transfer and the time slot in which it is to occur. When the circuitry connected to the idle line functions properly, the controller proceeds to the next idle line. If a malfunction is detected, the controller sets an alarm.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject matter of this disclosure is related to the subject matter of the following copending patent applications: Ring Scheduling Apparatus For A Digital Multiplexed Telecommunication System, Ser. No. 204,298, Filed on Nov. 5, 1980; and FIFO Buffer For Use In A Multiplexer of A Digital Multiplexed Telecommunication System, Ser. No. 204,299, Filed Nov. 5, 1980.

BACKGROUND OF THE INVENTION

As the cost of copper and copper cable rises and the population of the world increases, it is becoming more important for telephone companies to avoid installing new telephone cables to meet increased demands for service. High costs of new cable, new repeaters, new poles, and new central office switching circuitry would be incurred. In addition, labor costs in installing new cable on new poles and tearing up the streets and highways for new underground routes would drive costs still higher. All these new costs would necessitate rate hikes with the ultimate cost being borne by the telephone subscribers.

Accordingly, telephone utilities have sought ways to stretch their already existing facilities by such techniques as digital time division multiplexing and frequency multiplexing so that larger numbers of telephone users can all share the fixed number of circuits available in the cables already in place. Frequency division multiplexing involves transmitting the outgoing speech signals of a telephone line on one frequency and receiving the incoming speech signals for that line on a different frequency. Each telephone line then operates independently on its own pair of carrier frequencies, which pair of frequencies is different from all the other pairs of frequencies assigned to the other telephones sharing the common link. All the frequencies then are transmitted over the common link with frequency selective filters associated with each telephone such that no interference between the individual conversations occurs.

The multiplexing system of the invention uses multiplexing whereby each conversation is superimposed on the shared span line or assigned a particular time slot on a common circuit in a cable, which time slot is different from the time slots assigned to all the other conversations traveling down the common circuit "simultaneously". The analog or varying amplitude voice channels comprising the transmit and receive portions of each telephone conversation are converted to digital signals comprised of binary words or bytes. Each byte is a predetermined number of bits in length. Each combination of logic ones and zeroes in each byte represents a different amplitude level of the speech signal during a particular sample period. That is, each varying amplitude speech signal is chopped into a number of pieces of the same duration, which duration is called the sampling period. An analog-to-digital converter then converts the amplitude of the speech signal during each sampling period to a digital word in a code which represents that amplitude. For example, if an eight-bit word is used with each bit either a one or a zero, there are 256 distinct combinations which can be made. Each combination represents a specific amplitude level of the speech signal. These data words are then transmitted to the other party over the common circuit during the assigned time slot whereupon circuitry at the other end reconverts the digital words back to analog signals and transmits them to the earpiece of the second telephone. A similar circuit transmits the speech signals from this second telephone back to the earpiece of the first telephone during the same time slot.

Each common link circuit is called a span line, and each span line is comprised of two pairs of wires, one pair for each direction of transmission. Each span line in the disclosed system normally handles sixty-four telephone lines, and there are two span lines in the system, such that one-hundred-and-twenty-eight users can be coupled to the central office over only four pairs of wires. When one span line breaks down, all 128 users are coupled to the remaining span line.

Because span lines cover large distances and cause losses such that signals become weaker as they travel down the line, it is necessary to insert repeater amplifiers in the line to periodically boost the signal levels. Thus, it is advantageous to use digital multiplexing because it is easier for the repeaters to detect binary ones and zeroes than analog signals. As long as the repeaters can distinguish between a one and a zero, the distance of transmission has no effect on received signal quality. Also, identical circuitry is used as opposed to frequency multiplexing where different frequencies require different frequency selective filters.

The present invention pertains generally to the field of digital multiplexed telephone systems and, more particularly, to the circuitry for self-testing the audio channels, A/D and D/A converters, filters, and buffers which make the conversions.

SUMMARY OF THE INVENTION P The present invention provides a self-diagnosing telephone terminal for use in a digital multiplexed telephone system utilizing an apparatus for generating and detecting a digital representation of an analog test signal. The terminal has means which automatically monitors and tests each idle audio channel or line to insure that the circuitry connected therewith for transmitting analog signals and the digital representations thereof is operative.

The self-test circuitry includes a data bus for carrying digital representation of a predetermined analog test signal. This data bus is coupled to a generator which generates a plurality of digital words which describe, in combination, the analog test signal which will be used for the self-test function. The data bus is also coupled to a detector which detects whether there is digital data on the data bus which describes the test signal. There is also circuitry means for selectively enabling the generator and the detector, such that each can be selectively coupled to the data bus to automatically test idle circuits within the terminal.

This self-test circuitry works in conjunction with the time division multiplexing system as follows. A multiplexer is used to divide time on the internal data bus into a number of time slots for carrying the various voice channels. During each time slot, the multiplexer establishes a data link for high-speed pulse code modulated data being transferred between a source and destination storage register. Typically, the communication system is used for establishing telephone line data links between a central office and a large number of subscribers using only a two-pair T1-type span line. However, a T1-type span line need not be used. Other possibilities include a microwave link or an optical fiber-type link.

Regardless of what type of link is used between the remote terminal and the central office terminal, the multiplexers in each terminal convert serial data received from the link to parallel, high-speed digital data for transfer to various telephone lines connected to the terminal. A controller, incorporating a programmed microprocessor, controls the source and destination of each data swap performed by the multiplexer during each time slot. One of the swaps which is possible is for digital data from and to the self-test circuitry through the multiplexer to the line circuitry of a particular telephone line which is not being used. This line circuitry is responsible for converting the analog voice signals from the telephone into digital data and vice versa.

The self-test function requires that the controller determine which telephone lines are idle and then send a control word to the multiplexer to establish a high-speed data link between the tone generator through the multiplexer and to the line circuitry of the idle line and then back to the detector of the self-test circuitry. In this way, it is possible to send a predetermined test tone described by digital data to the line circuitry, have it converted to audio signals, then reconverted to digital signals and sent back through the multiplexer to the detector. The controller then tests the status of the detector and determines whether or not the circuitry connected to the idle line is functioning properly.

DESCRIPTION OF THE DRAWINGS

These, and other advantages of the present invention, are best understood through reference to the drawings, in which:

FIG. 1 is a block diagram of the digital multiplexing communication system;

FIG. 2 is a block diagram of the line circuitry for one line of a typical line card in the central office terminal;

FIG. 3 illustrates the difference between the line interface circuitry for a line circuit in the remote terminal and a line circuit in the central office terminal;

FIG. 4 is a block diagram of portions of the controller;

FIG. 5 is a block diagram of the multiplexer;

FIG. 6 is a block diagram of the self-test circuitry of the maintenance card;

FIG. 7 is a block diagram of the one-kilohertz detector circuitry of the maintenance card;

FIGS. 8A and 8B are a circuit diagram of the one-kilohertz detector 218 of FIG. 6 and the one-kilohertz tone generator;

FIG. 9 is a circuit diagram of the one-kilohertz LBU detector circuit forming part of the one-kilohertz detector;

FIG. 10 is a block diagram of the ring generator card;

FIGS. 11 and 12 are a logic diagram of the circuitry of the ring generator; and

FIGS. 13A and 13B together comprise a logic diagram of the re-frame buffer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a block diagram of the major functional components in either the remote terminal or the central office terminal of the digital time division multiplexing system which incorporates the circuitry of the present invention.

The audio signals to and from the telephones of the subscribers enter and leave the system on tip and ring pair groups 20 and 22. Each group of four telephone lines, i.e., eight wires, is coupled to one line card of which cards 24 and 26 are illustrative.

These line cards serve to interface the tip and ring telephone lines to the digital multiplexer system by performing several functions. First, they provide a status indication on whether any of the telephones connected to the line card are off-hook or dialing. The line card also amplifies the audio signals, converts outgoing audio to digital data and converts incoming digital data to audio signals, filters out high frequency components and other undesirable elements caused by these conversions and stores or buffers the signals for use by the rest of the system.

The line cards are coupled to the eight-bit parallel PCM main bus 28 and a PCM alternate bus 30. These buses carry the pulse code modulated digital signals to and from the multiplexers 42 and 44. The alternate bus is for reliability purposes, i.e., should one side of the system fail, its other side can then service the lines coupled to the failed side. The line cards are also coupled to the eight-bit parallel data main bus 32 and data alternate bus 34. These data buses carry digital status and control information to a pair of controllers 36 and 38 which control the two halves of the system, as will be more fully discussed below.

Controller A 36 and controller B 38 each contain a programmed microprocessor in the preferred embodiment. These controllers independently and asynchronously control the A and B halves of the terminal and each has two principal functions: call processing and maintenance supervision. The arrangement shown in FIG. 1 is used both at the remote terminal which interfaces with the actual telephone lines being serviced, and at the central office terminal which multiplexes the calls on the lines from the central office onto the span line coupled to the remote terminal- The controllers at both ends of the system have the same structure, but there is a difference in controller operation between the two ends of the system. For example, the controllers at the remote terminal end have a call processing function which includes checking the status of all the line cards via data buses 32 and 34 to determine which telephones are off-hook and which telephones are not in use. The remote terminal (RT) controller's call processing function also includes detecting pulse dialing on a dial-type telephone and transmitting a code indicating the number dialed to the central office (CO) controller which then causes a dial pulse relay connected to the proper line going to the central office switching apparatus to recreate the number dialed by suitably shorting together the tip and ring lines of the line to the central office.

The CO controller call processing functions are to allocate a time slot to a particular telephone user who is receiving a call or who is making a call. The CO controller also cancels time slots for terminated calls. In addition, the CO controller detects what lines from the central office are ringing and what frequency and DC polarity ring signal is present and sends a message to RT controllers indicating what lines are ringing and containing ring codes designating the frequency and polarity of the ring signal to be applied to the line being called. Both controllers at both ends engage in their maintenance supervision functions when not busy doing call processing operations. Basically, this maintenance supervision involves the automatic self-testing of the operational status of the line cards including alarm and display functions indicating when and where failures have been found.

Controller A 36 is coupled to all the line cards by control and address bus A 46 and data main bus 32. Controller B 38 is coupled to all the line cards by control and address bus B 48 and data alternate bus 34. Other individual control lines from both controller A 36 and controller B 38 are coupled to various circuits in the system to coordinate functions as needed.

Multiplexer A 42 serves to do the digital switching function under the control of controller A 36. For example, when controller A 36, assuming it is a CO controller, wants to connect a particular line for a telephone being called to the central office line which it finds ringing, it will scan for a free time slot and assign the time slot to the subscriber being called. There are twenty-three possible time slots of the twenty-four time slots on a span line in the system. The twenty-fourth time slot is used for supervisory message transmission between the CO and RT controllers. After the CO controller assigns a time slot, it signals the RT controller during the twenty-fourth time slot which telephone is being called and which time slot has been assigned. Controller A 36 will then send a control word over bus 51 to MUX A 42 telling it what exchange to make and during which time slot to make it. Bus 51 includes data, address, and control lines of controller A 36.

MUX A 42 is fundamentally a limited-scope, high-speed computer which uses the control word from controller A 36 as its program. The eight-bit, parallel digital data coming from and going to the particular telephones in use pass back and forth from the line cards involved via PCM main bus 28 with each transfer to and from a particular line card buffer occurring during the assigned time slot for that conversation. Multiplexer 42 contains a clock oscillator and counter which counts out the multiplexer's internal time slots. There are two multiplexer internal time slots for each span line time slot. Each multiplexer internal time slot has a read cycle and a write cycle. The control word received on bus 51 is stored in a memory which stores the control words for each internal time slot. The control word for each time slot is decoded in the multiplexer to tell it what source and what destination are involved in the transfer for that particular time slot.

Two transfers are made during each internal time slot. During the write cycle of a particular time slot, the multiplexer 42 will move the data to be sent to the telephone assigned that time slot from a source register, such as the span line receiver, to the line card buffer. During the previous read cycle, the binary data stored in the line card buffer coupled to the telephone line assigned that time slot is loaded onto the PCM main bus 28 and read into an input buffer in MUX A 42 coupled to the PCM main bus 28. During the write cycle of the next internal time slot associated with the single time slot assigned on the span line, the data in the input data buffer will be loaded in a destination register, such as the transmitter inside MUX A 42. During the read cycle of this next internal time slot, the data in the output buffer of the MUX will be loaded onto PCM main bus 28 and loaded into the buffer of the line card coupled to the telephone to which the data is being sent.

The timing/control signals to be sent to the line cards to control these transfers are generated in internal control timing circuitry coupled to the clocl oscillator in MUX A 42.

The address signals which are needed to select the line cards and individual line buffers involved in these transfers are generated in internal source/destination decoder circuitry coupled to the control word memory holding the control words for each time slot.

The aforementioned timing/control signals and address signals are sent to all the line cards via PCM address bus A 50 and clock and timing bus 146. Similar functions are performed by MUX B 44, PCM address bus B 52, and clock and timing bus 196.

Both multiplexers 42 and 44 convert the eight-bit, parallel digital data received on the PCM main and alternate data buses to a serial bit stream in NRZ, i.e., non-return-to-zero, code. This serial bit stream is transmitted to span terminating cards A and B, 54 and 56 respectively, on lines 58 and 60 respectively. Likewise, multiplexers 42 and 48 receive serial NRZ code from span terminating cards A and B and convert this serial data to parallel PCM data in NRZ code for transmission on PCM main and alternate buses 28 and 30. Both multiplexers 42 and 44 also supply a clock signal on lines 62 and 64 to span terminating cards 54 and 56 for use by those circuits if these multiplexers are functioning in the central office terminal, as will be described more fully below.

Span terminating cards A and B, 54 and 56, serve to interface between the multiplexers and the T1-type repeatered span line, as described in Bell System and AT&T specifications available to the public. Since this type of repeatered line is not designed to transmit digital signals in NRZ code, i.e., unipolar code, the span terminating cards combine the NRZ code and the clock signal from the multiplexers into a bipolar code suitable for transmission out on a T1-type repeatered line. In the other direction, the span terminating cards convert received bipolar data into unipolar NRZ code. Each span terminating card has an output for coupling to a transmit pair of a T1 line and an input terminal for coupling to a receive pair of a T1 line. The span line cards of the central office terminal also provide current feed to the repeaters.

Interconnect card 66 serves to help the system during heavy traffic conditions. Each of the A and B halves of the system can only accommodate twenty-three conversations at any particular time because that is all the time slots that are available on a T1 line with twenty-four time slots. When all twenty-three time slots in either half of the system are filled, the twenty-fourth user on that half of the system who is calling or being called will be shut out by an equipment busy signal. The interconnect card 66 serves to provide an overflow path for these shut out users to the other half of the system if there are unused time slots on that side. Likewise, when one side of the system fails, all the users connected to that half of the circuit are coupled to the other half. The interconnect circuit 66 aids in this transfer. This circuit is coupled to PCM main and alternate buses 28 and 30, the data main and alternate buses 32 and 34, CPU control and address buses A and B, 46 and 48, PCM address buses A and B 50 and 52, and clock and timing buses 146 and 196. The transfer function is under the control of controllers 36 and 38.

The circuitry of the line cards, multiplexers A and B, 42 and 44, controllers A and B, 36 and 38, span terminating cards A and B, 54 and 56, and interconnect circuit 66 is not critical to the invention and any circuitry which accomplishes the functions described herein will be adequate. The design and construction of these circuits is well within the capability of any designer of digital telecommunication systems in the exercise of ordinary engineering skill given a knowledge of the functions of these circuits as described herein. Detailed description of some of the circuitry will be omitted for the sake of brevity so that the true gist of the invention will not be lost in excessive detail.

Maintenance card 40 is shared by both halves of the system and is coupled to both the PCM data buses 28 and 30, both CPU data buses 32 and 34, both control and address buses 46 and 48, and both PCM address buses 50 and 52 and both PCM clock and timing buses 146 and 196. The maintenance card has circuitry to generate a one-kilohertz test tone in paralle, digital, eight-bit word format with eight digital words comprising the digital amplitude description of a single cycle of a one-kilohertz sinusoid. This digital data can be addressed by the multiplexers 42 and 44 under the control of controllers 36 and 38 as a source of digital data to be sent to a particular line.

The maintenance card 40 also includes circuitry to detect the presence of a one-kilohertz tone or, more accurately, the digital representation thereof, and to indicate the presence or absence of such a signal.

The self-test feature of the system described herein involves the controller, the multiplexer, the line cards, and the maintenance card 40. Each half of the system works in similar fashion so only the A side will be discussed. Controller A 36 constantly scans the line cards for their status by sequentially addressing the status registers of each line card via control and address bus 46. The status word in each register indicates the on-hook or off-hook condition of each line coupled to the line card, which status word is read into memory in controller 36 via data main bus 32. Controller 36, by virtue of this call processing function, also knows which lines are idle. A self-test program for the CPU in controller A sequentially tests the audio link circuitry of each idle line by sending the proper control words to the multiplexer 42. This control word causes multiplexer 42 to obtain a byte of the one-kilohertz test tone from maintenance card 40 and send it to the proper line card and the proper input buffer thereof for the idle line to be tested. These control words also direct the multiplexer to send the data received from the line card back to the one-kilohertz detection circuitry on maintenance card 40. Maintenance card 40 also includes controller addressable alarm and display circuitry for indicating when and on what line a failure has occurred.

Referring to FIG. 2, there is shown a typical line circuit for one of the four telephone lines coming from the central office to a line card of the central office terminal. The circuitry of FIG. 2 is found on each line card 24 or 26 in FIG. 1. Each line card interfaces with four telephone lines, and the circuitry shown in FIG. 2 is the circuitry necessary to interface with one line. Altogether 128 telephone lines are coupled to the line cards shown in FIG. 1. A ring detector circuit 72 senses the electrical condition of node 74. Node 74 is coupled to tip line 68 by resistor 76 and capacitor 78, and to ring line 70 by capacitor 80 and resistor 82. When that particular telephone line is being rung, a voltage will appear on tip and ring lines 68 and 70 sufficient to change the state of ring detector 72. Ring detector 72 then sets a latch in line status register 84 via line 86 which indicates that the line is being rung. Line status register 84 is continuously and periodically scanned by controller 36 or 38 (in FIG. 1) in the central office terminal. The controllers continuously scan all the status registers for all the line cards by selecting them through control and address buses A or B, 46 or 48. The data in the status register is then read over data main or alternate buses 32 or 34 and the status of all the lines connected to the line cards is determined.

When ringing is detected on the telephone lines 68 and 70 coming from the central office, the controller 36 or 38 of the central office terminal sends a message over the twenty-fourth supervisory time slot on the span line to the controller in the remote terminal telling it which line is ringing. When the RT controller (not shown, but similar to that shown in FIG. 1) receives this message, it addresses the proper line card (containing the circuitry of FIG. 2 with the line interface circuitry shown in FIG. 3) and the proper status register for the selected line via its address and control bus 46 or 48 and sets, with its data bus 32 or 34, a ring code in the status register for that line. This ring code tells the line card which line to connect to the ring generator and whether to connect the tip or ring line to the ring signal from ring generator 88 (in FIG. 1) so that ringing of the proper telephone will start. Meanwhile, the RT controller has enabled the ring generating card, 88 in FIG. 1, to cause it to generate a ringing waveform and has sent the proper control word to establish the frequency of ring signal to be generated and the proper polarity of DC voltage upon which the periodic portion of the ring signal is to be superimposed. This aspect will be discussed more fully in connection with FIGS. 10, 11, and 12. Ring generator card 88 is coupled to both the data main and alternate buses 32 and 34 and to control and address buses 46 and 48. It is also coupled to all line cards by its ring signal line 90.

FIG. 3 shows the different circuitry used to couple the telephone lines coupled to the remote terminal to the D/A and A/D circuitry of the remote terminal line cards. This latter circuitry is the same as shown in FIG. 2. Referring to FIG. 3, the addressed line card, when it detects a ring code in its status register, will signal ring relay circuit 92 to connect the ring signal to the proper line via contacts 94 or 95. Ring relay circuit 92 also opens contacts 96 and 97 during ringing so that the ring signal is not fed into transformer 108. The action of the ring relay will be inhibited if loop current detector 88 signals the line status and supervisory circuitry 84 via line 100 that the telephone is off-hook. Ringing will also be inhibited immediately when the telephone user picks up the ringing telephone. Ring trip detector 102 detects this condition and signals the supervisory circuitry 84 via line 104 to inhibit ringing. The controllers can also inhibit ringing when both controllers in a terminal have failed. Ring trip detector 102 couples the ring signal to the ring line 118 through relay contact 94 and provides a fast inhibit action via line 104 to the supervisory circuitry until loop current detector 88 takes over and indicates an off-hook condition to inhibit ringing. The circuitry of the ring trip detector and the loop current detector 88 is not critical to the invention.

The remote terminal line cards also indicate to the RT controllers that the user is off-hook and pulse dialing. This is done by loop current detector 88 which is coupled between terminal 106 of primary winding 112 of transformer 108 and ground. Tip line 110 is coupled to terminal 106 through transformer winding 112. Loop current detector 88 senses the DC component of the total loop current flowing through the tip and ring circuit from current source 114. This current source supplies "talk battery" current through transformer winding 116, ring line 118, the user's telephone, and back through tip line 110, winding 112, and loop current detector 88 to ground. This follows from the fact that there is a very high impedance seen between tip and ring lines 118 and 110 when the telephones coupled to these lines are on-hook but a finite impedance is seen between tip and ring lines 110 and 118 when any user coupled to these lines comes off-hook. Capacitor 118 provides an AC path for the audio signals between windings 112 and 116 while blocking DC current. Current source 114 is turned on by the controller 36 (in FIG. 1) when the user goes off-hook by a connection not shown.

The loop current detector signals the controller through line 100 and the status and supervisory circuitry 84 when pulse dialing is occurring as follows. The loop current detector senses when current is flowing in the tip and ring lines during the "make" portion of the pulse and sets line 100 to one binary state. During the "break" portion, line 100 is set to the opposite binary state. The controller reads line 100 via its data bus and determines what numbers are being dialed. It then sends this information, i.e., the line being dialed and the number dialed, to the controller in the central office terminal so that the line to the central office may be pulse dialed to recreate the number dialed. The circuitry of the line status and supervisory circuitry 84 is not critical and will be apparent to those skilled in the art.

Referring back to FIG. 2, when the CO terminal controller receives this dialing message, it addresses the line status register and supervisory circuitry 84 of the proper line card and the proper line and begins operating the dial pulse relay 118.

Two codes are involved in pulse dialing. A "dial pulse" code is sent by the RT controller during the make portion of the dial pulse and an "idle" code is sent by the RT controller during the "break" portion of the pulse dialing. When the CO terminal controller receives the dial pulse code, it causes relay 118 to close contact 120 and cause DC continuity between tip and ring lines 68 and 70 through transformer windings 122 and 124. Upon receipt of the idle code, relay 118 is caused to open contact 120. By making and breaking DC continuity through the central office tip and ring lines, the pulse dialing at the telephone is recreated. Touch tone dialing audio tones are transmitted through the regular audio circuitry of the line cards, as described more fully below.

The audio signals being transmitted between the central office and the telephone are coupled from tip and ring lines 68 and 70 through two-wire to four-wire conversion transformer 126. Signals coming from the CO are coupled through windings 122 and 128 to the input of amplifier 130. The output of amplifier 130 is passed through a fifth-order eliptical configuration transmit filter 132 of the sampled data type to remove frequencies that are higher than one-half the sampling rate of eight kilohertz. Signals higher than four kilohertz would cause in-band components in the sampling process and cause speech distortion. Low frequency rejection is also provided by this transmit filter to avoid low frequency return signals.

The output of transmit filter 132 is coupled to the A/D input of a PCM encoder 134. This encoder converts analog voice or test tone signals into standard eight-bit encoded PCM digital data with MU-255 companding. This is a standard PCM encoder utilizing an internal structure of a low pass filter on the input line, and a sample and hold circuit with its output going to the positive input of a comparator. The output of the comparator goes to a successive approximation register with an eight-bit parallel output coupled to a digital-to-analog converter. The D/A converter has its analog output coupled to the negative input of the comparator. The eight-bit parallel bus is then also fed through a parallel-to-serial converter. The sample and hold circuit provides the analog value of the input signal as it was at sampling time and holds it for the conversion cycle. Successive approximations are made with the output of the sample and hold circuit. The approximations are made by first turning on the most-significant bit at the input of the D/A converter causing it to output its highest voltage. The output is compared and if it exceeds the output voltage of the sample and hold circuit, the bit is turned off. If not, the bit is left on and the next bit is turned on. The process is continued until the least-significant bit is determined. The bits are then converted to serial format.

The PCM decoder 136 on the receive side of the circuit consists of the same circuits in reverse order. The output analog voltage is sampled and held then filtered to remove any quantization noise by filter 138. The output of filter 138 is amplified in amplifier 140 and transmitted through winding 142 of transformer 126 to tip and ring lines 68 and 70 via inductively coupled winding 124.

In the system disclosed herein, the sampling occurs 8,000 times per second and eight bits are used to quantize. The twenty-four voice band channels are received and transmitted simultaneously over a single two-wire pair in serial format at a rate of 1.544 megabits per second. The MU-255 companded pulse code modulation for T-carrier transmission means that low level analog amplitudes are encoded with small steps and high level signals with larger steps. This results in instantaneous companding following a logarithmic curve. Thus, each digital number represents the logarithm rather than the linear value of the sampled amplitude. This gives the voice quality transmission capability of twelve-bit linear PCM conversion.

The circuitry of amplifiers 130 and 140, filters 132 and 138, PCM encoder 134, and PCM decoder 136 are not critical to the invention and will be apparent to those skilled in the art given their functions as described herein.

The serial digital data is loaded on line 147 into registers in PCM digital I/O buffer 144 under control of the timing and control signals received on main PCM clock and timing bus 146 from multiplexer A 42 in FIG. 1. These timing and control signals indicate when the time slot assigned to a particular transfer of data is occurring and control the transfers during that time slot. There is one eight-bit register in buffer 144 for each telephone line coupled to a particular line card. The PCM data is transferred to the selected multiplexer on the appropriate PCM main or alternate bus 28 or 30 when the register for the particular line involved is selected by signals on the PCM address bus A or B, 50 or 52. Conventional selection circuitry under the control of the controllers 36 and 38 determines whether the line card is coupled to the main or alternate PCM data bus 28 or 30 and to CPU data bus 32 or 34 and to PCM address bus A 50 or B 52 and to CPU address and control bus 46 or 48.

FIG. 4 shows a block diagram of a structure for controllers 36 or 38 in FIG. 1 which would be satisfactory. There is nothing critical to the invention about this particular structure. Referring to FIG. 4, the status words from the A side line cards are loaded from the line status registers and supervisory circuitry 84 in FIG. 2 by controller A 36 into RAM 146 via data main bus 32. This scanning and loading is accomplished by CPU 148 under control of a program stored in ROM 150, relevant portions of which program are disclosed below in more detail. In the preferred embodiment, CPU 148 is a Z80™ microprocessor as it existed as of the filing date of this disclosure, manufactured by Zilog™ Incorporated or any second source of Zilog™. The operation of CPU 148 is conventional in that the program instructions and data transfers are accomplished by transfers on data main bus 32 to and from the address written on address bus 152 under the control of various conventional input and output control signals on control bus 154. RAM address decoder 156 and ROM address decoder 158 decode addresses on address bus 152 and control the locations accessed in RAM 146 and ROM 150 via lines 160 and 162, respectively. I/O address decoder 164 decodes addresses on address bus 152 and combines the various control signals on control bus 154 into control and address bus A 46. This bus is coupled to various other circuits in the system, as described herein, and serves to allow the various other circuits to be enabled when controller A 36 needs to write data to or read data from the selected circuit via data main bus 32. Control and address bus 46 also serves to control the timing of the transfers. There is nothing critical to the invention about the particular arrangement of the controller or the particular coding of the program disclosed herein.

Turning now to FIG. 5, which shows one possible structure for the multiplexers in the system, the operation of these multiplexers will be described. The specific circuitry used to accomplish the multiplexing function in the system is not critical to the invention. The description of the structure and functional relationship between the various internal portions of the multiplexer given here is intended only for the purpose of disclosing the best mode for combining the functional elements known at the time of filing. Any other equivalent combination of subcomponents that will accomplish the purpose and functions, as described herein, is intended to be included within the scope of this disclosure.

Multiplexer A 42 counts out the time slots for digital data transfers between various telephone lines in the system and either the line transmitter or receiver connected to the span line terminating circuit, or the maintenance card 40 in FIG. 1. Another multiplexer at the central office terminal takes the data received from multiplexer A 42 during a time slot on the span line and transfers it to the proper line coming from the central office main distribution frame equipment. The transfers of data to and from the maintenance card and the line cards are for self-test purposes of some of the audio circuits on the line cards coupled to idle telephone lines.

Multiplexer A 42 establishes an eight-bit, parallel, high-speed digital data link during each assigned time slot between the telephone lines in use or being tested and the proper source and destination registers. These links are established under the control of controller A 36 in FIG. 1 by way of control words written into four registers in channel link controller 166. These transfers occur over data main bus 32 and are controlled by control and address bus A 46. These two buses are combined into bus 51. Each control word controls the transfer during a single internal time slot.

During each time slot on the T1 span line, there are actually two internal transfers that take place: one between the line card and the MUX 42; and the other between the MUX 42 and the source and destination registers. There is nothing critical about the use of twenty-three or twenty-four time slots on the T1 span line and any other number which will work is permissible.

Multiplexer A 42 has a 1.544 megahertz clock oscillator 172 if it is in the central office terminal. If the MUX is in the remote terminal, the clock signals are received on the incoming clock line 62. These clock signals are derived from the RT incoming data on the span line by span terminating card A 54 in FIG. 1. The A side MUX at the central office terminal generates the master timing clock and the B side central office MUX is slaved to it so that they run synchronously until the master fails, at which time the slave runs on its own clock. The remote terminal MUX circuits are slaved to the incoming clock signals on line 62.

Line receiver 174 receives serial data in NRZ code on line 58 from span terminating card A 54 in FIG. 1 at a 1.544 megabits per second rate. It converts this data to parallel format digital output data and a time slot count for each eight bits and sends this data to re-frame buffer 178 via internal PCM bus 176. In the case that the line receiver 174 detects a loss of frame sync by way of detecting a frame bit error within three to five frames of the previous frame bit error, it signals the controller in FIG. 1 by writing an error status word into status register 175 via bus 177.

Data format in the preferred embodiment consists of twenty-four eight-bit words and one frame bit for a total of one-hundred-and-ninety-three bits per frame. The frame rate is eight kilohertz, i.e., eight thousand frames occur in the span line data every second. The frame bits alternate every other frame, and frame synchronization is accomplished by finding these frame bits.

The re-frame buffer 178 serves to establish synchronization between the received data and the data on the internal PCM data bus 180. This sync is necessary because delays in propagation of the data over the span line results in a phase difference between the received and transmitted data. In the preferred embodiment, the re-frame buffer receives the time slot count on bus 210 from control timing 184, and stores the eight-bit parallel data and time slot count received on bus 176 at the top of a thirty-two word First-In-First-Out buffer.

Data is loaded from the FIFO buffer onto internal PCM data bus 180 when the time slot count for the word at the output of the buffer equals the time slot that is presently being transferred on the PCM bus as indicated on line 210. This comparison is done by a comparator in re-frame buffer 178.

Control timing circuit 184 is coupled to clock oscillator 172 by bit timing line 186. Eight pulses occur on line 186 for each time slot on the span line. Clock oscillator 172 is coupled to line receiver 172 by bit timing line 190 and frame timing line 192 and to line transmitter 188 by bit timing line 194. The purpose of these connections is to keep clock oscillator 172, line receiver 174, line transmitter 188, and control timing 184 in synchronization with the master clock. The control timing circuit 184 also receives a frame reset signal on line 185 from line transmitter 188 to reset its time slot counter after twenty-four time slots. The frame timing reset signal on line 185 is generated by arbitrarily starting a count at any bit time and occurs after every one-hundred-and-ninety-third bit. It serves as the framing pulse at the end of the span line to which the multiplexer is coupled. This line to which the multiplexer is coupled. This frame timing pulse is independent of the frame timing pulse at the central office terminal multiplexer and is independent of and asynchronous to the frame timing pulse from line receiver 174. This frame timing pulse is used to slave the re-frame buffer 178 so as to cause it to release incoming data for a time slot on the span line during the corresponding time slot for line transmitter 188. This insures that bidirectional transfers to and from the line cards are synchronized. The re-frame buffer 178 permits the incoming and outgoing data transfers to be synchronized within the terminal regardless of time delays caused by the length of the span line. The control timing circuitry 184 generates the main timing signals for the MUX circuitry. It also generates PCM bus clock and timing signals on bus 146 to control PCM data transfers to and from the line cards and maintenance card.

Control word RAM 198 has eight bits of its data bus coupled to source/destination register 165 and status register 175 in channel link controller 166 by bus 204. These same eight bits are also coupled to source/destination decoder 200 and temporary RAM 202. Link address register 201 and link time slot register 203 in channel link controller 166 and card address decoder 206 are coupled to the other eight bits of the data port of control word RAM 198 by operation register address bus 208. The output of card address decoder 206 is buffered and sent out over PCM address bus A 50 to the line cards and maintenance card 40 in FIG. 1.

There is one sixteen-bit word in control word RAM 198 for each of the forty-eight internal time slots used by MUX 42.

The control words are written into control word RAM 198 over source/destination bus 204 and operation register address bus 208 from the four, four-bit registers in channel link controller 166 during the write cycle of the time slot corresponding to a particular address in the RAM 198 to be updated. The address port of RAM 198 is coupled by line 210 to the time slot count from control timing 184. Control RAM 198 is constantly being read using the time slot count on line 210 from control timing 184 as the read address during every time slot. Its contents are decoded to determine which operation register and which source and destination registers are involved in the transfers to be made during each time slot. The decoded outputs enable the proper registers for reading and writing.

The CPU in the controller 36 (in FIG. 1) can address any one of four registers in the channel link controller 166 individually to change a control word. One register, 203 in FIG. 5, contains data on the time slot assigned to a particular exchange; another pair, 201 and 165, contain information as to the link address and the source and destination registers for a particular transfer; and the last is a status register 175 which may be read or updated by the controller at any time. The channel link controller can only update a word in the control word RAM 198 during the write cycle of a particular time slot. Also, the CPU must write to the link address register 201 or source/destination register 165 before writing to the link time slot register 203. A busy flag in the status register 175 will be raised when the time slot register is updated by the CPU. This flag will not be cleared until the channel link controller 166 has had an opportunity to update the word in the control word RAM 198 during the write cycle of the time slot corresponding to the new control word.

The internal PCM data bus 180 from re-frame buffer 178 is coupled to line transmitter 188, temporary RAM 202, and input and output buffers 168 and 170. The re-frame buffer is also coupled to line transmitter 188 by a loop data bus 181 which merely transfers data from the line receiver 174 back out through line transmitter 188 in the eventuality that no transfer is made by multiplexer 42 during a particular time slot for testing purposes. Input and output buffers 168 and 170 are also coupled to PCM main bus 28 from the rest of the system.

Operation of the multiplexer during a typical time slot assigned to a telephone conversation is as follows. The central office terminal controller will detect ringing on a line and send a message to the remote terminal controller via the twenty-fourth supervisory time slot indicating that a line ringing, which line it is and what time slot is assigned to it. The remote terminal controller will then start ringing on the designated line and send a control word to the MUX 42. The control word would put the code for the designated line in link address register 201 and the code for the line receiver 174 as the source and the line transmitter 188 as the destination in source/destination register 165. The controller would then write the code for the designated time slot into link time slot register 203. The next time this time slot count came up on bus 210, this control word would be written into the address in control word RAM 198 corresponding to that time slot via source/destination bus 204 and operation register address bus 208.

The next time that assigned time slot count occurred at the address part of control word RAM 198, the new control word would appear on source/destination bus 204 and operation register address bus 208. The eight bits on the source/destination bus 204 would be decoded and would enable line receiver 174 via select receiver line 205 and would enable line transmitter 188 via select transmitter line 207. The other eight bits on operation register address bus 208 would be decoded by card address decoder 206 into the address of the correct buffer on the line card coupled to the telephone line being called and would enable that buffer.

Then during the read cycle of the assigned time slot, the contents of the line receiver 174 will be loaded via internal PCM data bus 180 into output buffer 170. At the same time, the buffer of the line being called will be loaded via PCM main bus 28 into input buffer 168, all under the control of PCM clock and timing bus 146. During the write half of the time slot, the input buffer 168 will be loaded into the line transmitter 188 via internal PCM data bus 180, and the output buffer 170 will be loaded into the buffer for the line being called via PCM main bus 28.

DESCRIPTION OF THE SELF-TEST SYSTEM

FIG. 6 shows the structure of maintenance card 40 in FIG. 1. Turning now to FIG. 6, the operation of the maintenance card 40 will be described. Control and address bus A 46 from controller A 36 (in FIG. 1) is coupled to address decoder 212 and to one-kilohertz detector 218. Address decoder 212 is coupled to display and alarm circuit 214 via enable line 216 and enables these circuits when the address of the alarm and display circuits is detected on control and address bus 46. Decoder 212 enables one-kilohertz detector 218 via enable line 220 when the address of the detector appears on control and address bus 46.

Data main bus 32 (in FIG. 1) is coupled to display and alarm circuits 214 and to detector 218. PCM main bus 28 is coupled to both the detector 218 and the tone generator 222. Main PCM clock and timing bus 146 is also coupled to tone generator 222 and to detector 218. Display A 226 is coupled to display and alarm circuit 214 by line 228, and PCM address bus 50 (in FIG. 1) is coupled to tone generator 222.

When the controller A 36 (in FIG. 1) determines during its normal scan of its telephone lines that a line is idle, a self-test routine is entered. The program for this self-test routine is disclosed more fully below. To self-test the idle line, the controller sends a control word to multiplexer 42 (in FIG. 1) telling it to set up a link between the maintenance card and the idle line during time slots assigned for this purpose. The controller 36 will then enable the tone generator 222 via PCM address bus 50 (in FIG. 1) and clock and timing bus 146 (in FIG. 1). The tone generator generates the digital representation of a periodic test tone, one kilohertz in the preferred embodiment, by placing one of eight, eight-bit words on PCM main bus 28. These eight words represent the logarithmically compressed amplitude at eight sampling points of a one-kilohertz sinusoidal waveform. One eight-bit word is placed on PCM main bus 28 during each frame during the assigned time slot as determined by a control signal on main PCM bus clock and timing bus 146. The eighth bit is a sign bit and is derived from the most-significant bit of a counter which functions in the circuitry used to generate the eight digital words for the self-test feature. The eight digital words comprising the logarithmic description of one cycle are shown generally at 234 in FIG. 6. Inversion of the first seven bits starting at the LSB and conversion to analog yields the compressed version of the sinusoid where y=log (1+(MU)X)/log (1+MU) and MU=225.

Two internal MUX time slots are dedicated to the self-test function. One slot is for transfers between the MUX 42 (in FIG. 1) and maintenance card 40. The other time slot is for transfers between the line card being tested (in FIG. 1) and the MUX. There is a read cycle and a write cycle for each time slot. During the read cycle of the first assigned time slot, the first word of the one-kilohertz test tone will be loaded into the input buffer, 168 in FIG. 5, of MUX 42. Simultaneously, source/destination decoder 200 will have decoded the bits of the control word on source/destination bus 204 so as to enable temporary RAM 202 and will cause it to load its contents into output buffer 170. During the write cycle of the assigned time slot, the input buffer 168 will load its data via internal PCM data bus 180 into temporary RAM 202. Simultaneously, the contents of the output buffer 170 are placed on the system PCM main bus 28 and loaded into the detector 218 in FIG. 6. This completes the first transfer.

During the read cycle of the second self-test time slot, data from the buffer coupled to the line circuitry to be tested is transferred to the input buffer 168 and data from the temporary RAM 202 is loaded into the output buffer 170. During the write cycle, the data in the input buffer 168 is loaded into temporary RAM 202 and data from output buffer 170 is loaded into the register coupled to the line being tested. This completes the second transfer. Transfers such as these occur for one external time slot per frame until the controller 36 is satisfied that the line is working. The controller then picks the next idle line and assigns the same two time slots to test it.

Referring again to FIG. 2 showing the circuitry of the line cards in the central office terminal, the self-test feature tests the operability of the PCM digital I/O buffer 144, PCM decoder 136, receive filter 138, amplifier 140, winding 142, winding 128, amplifier 130, transmit filter 132, and PCM encoder 134. Because there is a very large impedance seen looking out from the tip and ring line when the subscriber is on-hook or the central office equipment is not ringing the line, the audio energy leaving PCM decoder 136 on line 137 cannot go out of transformer 126. Instead it propagates through receive filter 138, line 139, amplifier 140, winding 142, winding 128, and then back through the transmit side of amplifier 130, transmit filter 132, PCM encoder 134, and back into I/O buffer 144 as digital information which is read and conveyed back to the detector circuit 218 in FIG. 6.

Controller 36 (in FIG. 1) decides if the line is functioning properly by addressing detector circuit 218 via control and address bus A 46 (in FIG. 1) and reads the detector output via data main bus 32 (in FIG. 1). If a one-kilohertz tone is present, the line has passed. If no positive indication is received, the controller checks to see if the tone generator 222 is working by enabling detector 218 so that it is loading data from the PCM main bus 28 (in FIG. 1) when data is being transferred from the tone generator 222 to temporary RAM 202 in FIG. 5. If the tone generator is working properly, then the controller enables the detector when data is being transferred from the temporary RAM 202 to the line card. If temporary RAM 202 is working properly, then the detector 218 is enabled when data is being transferred from the line card to the temporary RAM. If this transfer fails, the controller sets an alarm by addressing and writing to display and alarm circuits 214 in FIG. 6 via data main bus 32. Display and alarm circuits 214 set an alarm relay 230 by line 232 and displays the location of the malfunctioning line on display A 226 via line 228.

FIG. 7 shows the structure of detector 218 in FIG. 6. FIGS. 8A and 8B show the detailed logic of the detector 218 and tone generator 222 in FIG. 6 when placed side by side. Turning now to FIGS. 7, 8A, and 8B, the operation of detector 218 and tone generator 222 in FIG. 6 will be more fully explained. The details of decoder 212, display and alarm circuit 214, display A 226, and alarm relays 230 are omitted for clarity since they are not critical to the invention and should be apparent to those skilled in the art.

Time slot counter 236 in FIG. 7 serves to count out the number of the time slot whose control word is presently controlling the MUX. It increments once for every time slot by sensing the conditions on PCM clock and timing bus 146. Its output is coupled to one input of comparator 238 via bus 242.

Comparator 238 in FIG. 7 has its other input coupled via bus 242 to the output of time slot register 244. This register has its data input coupled to data main bus 32 and it is addressed and controlled by signals on control and address bus A and decoded address line 220 from address decoder 212 in FIG. 6. This register serves to store the number of the time slot assigned for a particular self-test transfer to be detected so that this time slot number can be compared by comparator 238 to the time slot the MUX is in currently.

Bit seven from time slot register 244 on line 246 is coupled to the from/to CODEC control circuit 248. This circuit is also coupled to the output of comparator 238 via line 292, and receives control input signals from the PCM clock and timing bus A 146 from MUX 42 (in FIG. 1). From/to CODEC control circuit 248, under the control of bit seven from the controller, controls whether the detector is loading data from the PCM main bus 28 during the read cycle or the write cycle of the time slot stored in time slot register 244. This control is asserted through output line 252 which controls the time when flip-flop 254 is loaded from the PCM bus.

Flip-flop 254 loads only the sign bit of the PCM bus 28. The sign bit is the most-significant bit (MSB) of the eight-bit word which describes the amplitude of the test sinusoid at any particular time. As seen generally at 234 in FIG. 6, it is zero for the first half of the cycle, i.e., the first four words, and one for the last four words. Thus, the sign bit is a logic one exactly once during every cycle and will be high one-thousand times per second. The output of flip-flop 254 is coupled to the input of detector circuit 256 by line 258. The detector 256 senses whether there is a one-thousand hertz cycle of pulses on line 258 and indicates yes or no to the controller via line 260.

Turning now to the more detailed schematic of the one-kilohertz detector circuit 218, shown in FIGS. 8A and 8B, the time slot register 244 is comprised of octal flip-flop 262 having its data inputs coupled to the eight bits of data main bus 32. In the preferred embodiment, octal flip-flop 262 is a Motorola™ MM 74C 374. The low-order six bits at the output of time slot register chip 262 on bus 240 are applied to one compare input of comparator 238 which is comprised of comparator chips 262 and 264. In the preferred embodiment, these chips are Motorola™ MC 14585 B. Bit seven of the output from time slot counter 236 is applied via line 246 to FROM/TO CODEC control circuit 248.

The other side of comparator 238 is coupled via line 242 to the output of time slot counter 236. This counter has its count and reset inputs coupled to the main PCM clock and timing bus 146. Specifically, the count input which advances the counter on a high to low transition is coupled to the output of NAND gate 264. This gate has its inputs coupled to the PCM load I/O signal and the PCM R/W signal which are both signals in the PCM bus clock and timing bus 146. The PCM load I/O and PCM R/W signals are such that they are both high at the inputs of NAND gate 264 only once during each internal MUX time slot. This causes a high to low transition at the output of NAND gate 264. The A MSI signal from the PCM bus clock and timing lines 146 is normally low, and it resets the counter when it goes high at the end of every frame. The counter output is latched on bus 266 into hex flip-flop 268. The six outputs from latch 268 are the outputs of the counter on bus 242. Latch 268 is reset by the A MSI signal on line 270 rippling through inverter 272 and line 274 coupled to its reset input. Latch 268 is loaded once per internal MUX time slot by the PCM R/W signal of the PCM bus clock and timing lines 146 through line 276, inverter 278, and line 280 coupled to its clock input. In the preferred embodiment, counter chip 282 is a Motorola™ MC 14024 B seven-stage, ripple counter, and latch 268 is a Motorola™ MC 14174 B.

FROM/TO CODEC control 248 includes exclusive-OR gate 284, inverter 286, and NAND gate 288. The output of NAND gate 288 is coupled to the C input of flip-flop 254. One input of NAND gate 288 is coupled to the A=B output of comparator 238 by line 250. Another input is coupled to the output of inverter 286 by line 294. A third input is coupled to the PCM load I/O signal by line 295. The input of inverter 286 is coupled to the output of exclusive-OR gate 284 by line 298. This gate has a first input coupled to bit seven on line 246 and a second input coupled to the PCM R/W signal via line 300. Thus, the output on line 298 of exclusive-OR gate 284 will be high whenever line 246 or line 300 is high but not when both are high. PCM R/W is generated by control timing 184 in FIG. 5 to be high during the write cycle half of each internal MUX time slot. Line 246 is high whenever controller A 36 sets it high. Thus, line 298 will go high and line 294 will go low during the write cycle of each MUX time slot unless controller 36 makes line 298 go high during the read cycle of the time slot. If the latter occurs, detector 218 will be caused to load the sign bits from PCM main bus 28 during the read cycle instead of the write cycle.

The output of comparator 238 on line 250 goes high when the output of time slot counter 236 equals the time slot stored in time slot register 244. Flip-flop 254 only loads the sign bits on PCM bus 28 when its clock input coupled to line 252 is high. In the preferred embodiment, flip-flop 254 is a Motorola™ MC 14013 B. The output of NAND gate 288 is high at all times except when all its inputs are high. Thus, when the desired time slot occurs and PCM load I/O indicates that loading on the PCM main bus 28 is occurring, the PCM R/W signal on line 300 will cause line 294 to go low either during the read cycle or the write cycle of the MUX time slot depending upon whether the bit seven signal on line 246 is high or low. When line 294 goes low, line 252 goes high and flip-flop 254 starts loading sign bits.

Time slot register 244 is loaded from data main bus 32 when the signal on line 304 from the output of NAND gate 306 goes high. One input of NAND gate 306 is coupled to the original CPU I/O WRITE on line 308. Another input is coupled to the signal 1-kHz DETECT ENABLE on line 220. The signal on line 304 is high when either of lines 308 or 220 are low. The signal on line 220 is high whenever the detector 218 is enabled by address decoder 212 in FIG. 6. CPU I/O WRITE is active low, i.e., the controller asserts this signal low when it wants to write on data main bus 32. When this signal goes low, line 304 goes high and the time slot data on the data bus 32 is latched into octal flip-flop 262.

Because flip-flop 254 is latching sign bits at a one-kilohertz rate, its Q output coupled to line 258 will be changing at a one-kilohertz rate also. Level changing voltage divider resistors 314 and 316 condition the signal on line 312 for input to detector chip 256. The output node 320 of the divider is coupled to two 0.01-microfarad capacitors 332 and 334. The output of one of these capacitors is coupled to pin 1 of the detector chip 256, and the output of the other is coupled to pin 6 of the detector. A 316-kilohm resistor is coupled from pin 1 to pin 6. Pin 8 of the chip is coupled to ground through a one-microfarad capacitor 338 and pin 4 is coupled to line 260 which is the detector status bit which indicates whether a tone was detected or not.

FIG. 9 shows the structure of detector chip 256 in FIGS. 7 and 8. Turning to FIG. 9, the detector chip 256 will be described in more detail. This is a custom designed chip, but the circuitry other than as described here is not critical to the invention. Pin 1 is the input and is coupled to the negative input of differential amplifier 322. The positive input of this amplifier is coupled to a fixed voltage reference source 324 by line 326. This voltage is fixed at -3.8 volts above the supply voltage V- on pin 2. The output of differential amplifier 322, line 328, is coupled to pin 6 and to the input of unity gain amplifier 330. Capacitors 332 and 334 and resistor 336 in FIG. 8A coupling pin 6 to pin 1 of chip 318 form an active filter. Unity gain amplifier 330 is a buffer and has its output coupled to pin 8 which is coupled to ground by capacitor 338 in FIG. 8A. Capacitor 338 serves as a holding capacitor to form a peak detector. The voltage level at pin 8 is coupled to the positive input of comparator 340 which tests it against the fixed reference voltage 324 which is coupled to its negative input by line 326. The output of comparator 340 is coupled to pin 4 and line 260 in FIG. 8A.

Returning to FIG. 8B, the structure and operation of the tone generator 222 will be described. In the preferred embodiment, counter 342 is a Motorola™

MC 14520 dual, BCD, up-counter with its clock input and its reset input grounded. The enable input is coupled to the A MSI signal by line 270. The counter will increment on every downward transition of the A MSI signal, which occurs every frame, i.e., every one-hundred-and-twenty-five microseconds. Bits Q0 and Q1 are coupled to two inputs of exclusive-OR gate 344 which has its output on line 346 coupled to the input of latch 348. In the preferred embodiment, this latch is a Motorola™ MM 74C 374. The Q2 bit of counter 342 is coupled by line 349 to another input of latch 348.

The L1 and L2 signals on lines 346 and 349 comprise the only bits which are changing of the bits in the eight digital words depicted in FIG. 6 at 234. That is, a close study of this pattern reveals that bits two and four are always high or logic one and bits six and seven are always low or logic zero. Bit eight is the sign bit and is not involved. Bit five is the inverse of bit one which means only bit five and bit three need be generated.

This generation of bits five and three is done by counter 342 and exclusive-OR gate 344. During the first frame, word one at 264 in FIG. 6 will be generated. The outputs of counter 342 will be such that exclusive-OR gate 344 generates one of the two bits necessary on line 346 which is applied to pin three of latch 348 and the inverse to pins thirteen and seven. Bit Q2 of the counter will be the other bit on line 349 and is applied to pin eighteen of latch 348. All the other necessary bits are generated by connecting pins four and eight of latch 348 to a positive voltage source and pins fourteen and seventeen to ground. The next A MSI signal on line 270 to the counter 342 and to the latch clock input 348 will increment the counter and load the latch for the next frame. The outputs of the latch are coupled to the PCM main bus 28.

Latch 348 is enabled by the signal on line 350 from the output of NAND gate 352. This gate has one of its inputs coupled via line 280 to the output of inverter 278 in FIG. 8A which is inverting the PCM R/W signal. Two other inputs of NAND gate 352 are coupled to the Q output of flip-flop 354. This flip-flop has its C input coupled to line 280 and its D input coupled to the output of inverter 356. This inverter has its input coupled to the output of NAND gate 358. This gate has three of its inputs coupled to two bits of the PCM address bus 50 and a third coupled to the PCM maintenance card select signal on line 360 forming part of the PCM clock and timing bus 146. Flip-flop 354 also has its set and reset inputs grounded. In this configuration, the output of NAND gate 352 will go low enabling latch 348 during the read cycle of any time slot when the maintenance card is selected by a high on line 360 and when the tone generator 222 is addressed by MUX 42 by highs on PCM AD 1 and AD 2 lines. This would cause a low on line 362 and a high at the D input of flip-flop 354 which sees a high at its C input during the read cycle of the assigned time slot. Thus, the Q output is set and latch 348 is enabled.

DESCRIPTION OF THE RING SCHEDULING SYSTEM

FIG. 10 shows the structure of ring scheduling card 88 in FIG. 1. Turning now to FIG. 10, a more detailed description of the operation of ring generating card 88 in FIG. 1 will be given. Ring scheduling circuitry is necessary both to accommodate the different ringing systems employed by the Bell Telephone System versus independent telephone companies, and to accommodate the fairly limited amount of power available in remote terminals for ringing telephones by limiting peak ringing signal power consumption. Generally, a ringing telephone requires about one watt to ring the bell. If a large number of telephones are all ringing simultaneously, the power supply may be inadequate, leading to system failure.

The Bell System ringing scheme for party lines differs from the ringing scheme used by independent telephone companies. That is, on the Bell System party line up to four subscribers may be simultaneously attached to one line. Each telephone line is comprised of a tip line and a ring line. The ringing signals on any one party line can thus be directed to the proper phone being called by making the four individual telephones on the line each responsive to a different type ringing signal. That is, one phone may be made responsive to a ring signal having a first characteristic such as a 20-hertz signal superimposed upon a positive 48-volt DC level on the tip line. A second phone may be made responsive to a ring signal having a second characteristic such as a twenty-hertz tone superimposed upon a negative 48-volt DC signal on the tip line. A third phone may be made responsive to a twenty-hertz ringing signal superimposed on a positive 48-volt DC signal on the ring line. Finally, the fourth phone may be made responsive to a twenty-hertz ringing signal superimposed on a negative 48-volt DC signal imposed upon the ring line.

A different ringing scheme is utilized by the independent telephone companies for party lines in that the independents use different frequencies. That is, up to five phones may be simultaneously connected to a particular party line. Each phone on the line may be made responsive to a ringing signal of a different frequency, such as by band pass filtering. A duty cycle of one second on and four seconds off is imposed by the software on the ring generator shown in FIG. 10 for independent ringing. That is, all five telephones on a particular party line may be rung during a five-second interval, by imposing a different frequency ringing signal on the line during each one-second interval of the five total seconds in the duty cycle. This can be understood more clearly by imagining that all five telephones connected to a particular party line were in a particular house. If all five phones were called simultaneously, the ringer scheduling circuitry disclosed herein would generate a different frequency ringing signal during each second of the five total seconds of the duty cycle. Thus, the phones would all be ringing, but the actual ring portion of the familiar on-off ringing that would not occur in more than one phone at any particular instance in time. If all five phones were lined up in a row, phone 1 would ring during the first second, phone 2 would ring during the second second, phone 3 would ring during the third second, and so on, until all five phones had rung their first ring.

The Bell System requirements impose a different duty cycle on the ring generator for Bell System party lines. On Bell System party lines having four subscribers connected to each party line, only three of the telephones connected to each party line will be rung during any six-second interval. The software employed by the digital multiplexing system disclosed herein establishes a two-second on and a four-second off duty cycle when a switch on the CO controller card indicates the system is installed in a Bell System link. During the two seconds "on" of the duty cycle, only one of the aforementioned four combinations of DC polarity and tip or ring selection will exist on the tip and ring pair of the party line. That is, if for the first two seconds of the total of six seconds of the duty cycle, a twenty-hertz ringing signal is superimposed upon a 48-volt positive DC level imposed upon the tip line, then none of the other three conditions will be imposed upon that particular party line during those two seconds as that would cause a conflict in voltages on the line. Thus, during the first two seconds, only the telephone which is responsive to the positive 48-volt DC level on the tip line will ring. However, during the next two seconds of the duty cycle, a negative 48-volt DC level with the ring signal superimposed can be placed on the tip line or ring line. Of course the digital multiplexed telephone system disclosed herein is not designed to ring more than one phone on a party line at any one time since as soon as that phone is picked up, the other phones on the line cannot be used because the tip and ring pair can carry only one conversation at a time.

The same concept holds true however for any two-second portion of the duty cycle of the ring generator. Since the ring generator serves all the line cards which are coupled to all the telephone lines in the system, during any two-second portion of the Bell System duty cycle, or during any one-second portion of the independent telephone company duty cycle, the ring generator can only put out only one type of ringing signal. That is, for the Bell System-type ringing, the ring signal can only be superimposed on a +48 volt DC level or a -48 volt DC level. For independent ringing, the ring generator can generate only one frequency at a time. Because power is limited in the remote terminals, only a predetermined number of subscribers, three in the preferred embodiment, can be rung during any particular 2-second portion of the Bell System duty cycle. That is, the remote terminal is programmed to determine all the subscribers who are supposed to be ringing at any particular moment in time and schedule them into groups of up to three subscribers for each two-second portion of the six-second duty cycle. The controller is programmed to arrange these groups such that all three subscribers' lines assigned to a particular two-second portion of the duty cycle have similar ringing conditions, i.e., each of the three telephones assigned to a particular two-second group must be voltage compatible with the other two such that all respond to a +48 volt DC on either the tip or ring line or all respond to a -48 volt DC on either the tip or ring line. Only the polarity need be compatible since the line cards receive ring codes which determine whether the ring signal is applied to the tip or ring line.

Likewise, for the independent telephone company ringing scheme, the remote terminal controller is programmed to organize a schedule of all the telephone lines which are ringing at the central office at a particular time. The RT controller then assembles groups of up to three frequency compatible subscribers on separate party lines for each one-second portion of the five-second duty cycle. The screening for compatibility and scheduling of individual subscriber lines for which ringing is desired into groups is done by the controller under software control.

When the central office terminal controller detects ringing on a particular line, it sends a supervisory message to the remote terminal controller over the supervisory time slot which indicates which subscriber is being called and the particular ring code assigned to that subscriber's telephone. The messages are accumulated by the remote terminal controller in memory and organized into a table from which the ring scheduling is done by the ring scheduling subroutine of the program stored in the ROM in the controller. Once the sorting is accomplished by the remote terminal controller, the hardware of the ring scheduling system functions as follows.

Referring to FIG. 10, after sorting, the controller in the remote terminal generates an address and control signals for the ring generator on its control and address bus 46. These signals include the card select signal on line 364 and the WRITE I/O signal on line 390 which enable decoder 374. The controller then will take the results from its sort and determine for the first group and first portion of the duty cycle what frequency of ring signal is required and the polarity. The results of this decision, of course, depends upon whether the switch on the central office terminal controller indicates that Bell System ringing or independent telephone company ringing is desired.

After enabling decoder 374, the controller writes the proper data on its data bus to select either the positive or negative polarity for the DC voltage to be applied to the DC input of the secondary winding of the transformer. In the preferred embodiment, this code is written on bit D2 of data main bus 32. Bit D2 is coupled through buffer 366 and line 368 to the D input of flip-flop 370. Flip-flop 370 stores the polarity data from the CPU to control relay K1 372.

Controller A, 36 in FIG. 1, loads flip-flop 370 in FIG. 10 by generating the proper address signal on address lines ADR 1 and ADR 2 of address and control bus 46 in FIG. 10. When this address code appears at the input to decoder 374, the output on line 376 coupled to the clock input of flip-flop 370 causes that flip-flop to be loaded. This causes the Q output of flip-flop 370 coupled to line 376 and inverter 378 to place relay K1 in the proper state via line 380 such that contacts 382 assume the desired polarity position. Contacts 382 serve to couple the negative 48-volt DC or positive 48-volt DC outputs of a DC voltage source (not shown in FIG. 10) to the DC input 384 of secondary winding 386 of transformer 388. Decoder 374 is enabled to set this chain of events in action when the WRITE I/O signal on line 390 goes low. This signal is a control signal generated by the controller which indicates, when it goes low, that data is being written onto data main bus 32. The card select signal on line 364 is a high level signal when the ring generation card is selected. Thus, when the signal WRITE I/O goes low, inverter 392 presents a high level signal via line 394 to the input of NAND gate 396 to which it is coupled. NAND gate 396 drives its output line 398 low thereby enabling decoder 374 to decode the address bits on address lines ADR 1 and ADR 2.

Buffer 366 buffers data lines 1-4 so that only one load is seen on these lines by the data outputs of the controller 36 in FIG. 1. If buffer 366 was not present, two of these four data lines would be loaded by two loads instead of one, i.e., latch 400 and flip-flops 370 and 422.

The controller, after loading the polarity data, must load data into latch 400 which will determine the proper frequency of ring signals to be generated. This loading is accomplished by enabling decoder 374 and placing the proper address code on address lines ADR 1 and ADR 2 so as to drive line 402 coupled to the clock input of latch 400 to the proper state to cause latch 400 to load the data from data main bus 32 and latch it on its output bus 404.

The digital data latched on bus 404 causes divider 406 to divide the incoming clock signal on line 408 by a factor N, which factor is determined by the digital data comprising the control word on bus 404. For example, if the control word indicates that factor N is to be eight, the output signal from divider 406 on line 410 will be a pulse train having a frequency of 1,000 hertz where the incoming clock signal on line 408 is 8,000 hertz.

The output signal on line 410 is coupled to the input of square wave generator 412 which converts the irregularly-shaped pulses on line 410 to a square wave on output line 414.

Power down circuit 416 couples the output of square wave generator 412 to optical couplers 418 by line 420. Power down circuit 416 will disable the ring generator if the CPU valid signals on bus 422 indicate that both controllers in the remote terminal have failed. The power down circuit can also disable the ring generator upon command of the controller. This is accomplished by the controller by enabling decoder 374 and writing the proper address code on address lines ADR 1 and ADR 2 so as to drive line 420 coupled to the clock input of flip-flop 422 to a state such as to cause loading of the flip-flop. The data input of this flip-flop is coupled to data bit D3 of data main bus 32 by line 482. Thus, the controller can enable or disable the ring generator by setting bit D3 of data main bus 32 either to one or zero and addressing flip-flop 422.

Optical couplers 418 couple the square wave ring signal on line 420 to the input of push-pull drivers 424 and 426 by lines 428 and 430, respectively. Optical coupler 418 isolates the logic on its input side from the telephone line driving circuitry on its output side. Thus, if lightning strikes a particular telephone line and a voltage surge is coupled back along the line to optical coupler 418, there will be no direct electrical connection for this voltage surge into the CMOS logic circuitry on the input side of optical coupler 418.

Drivers 424 and 426 are darlington pair amplifiers having outputs coupled via lines 600 and 624 to primary windings 432 and 434 of transformer 388. Secondary winding 386 has one end coupled to line 384 which end is designated the DC input. The other end is coupled to line 90 through the relay contacts of relay K2 428 and is designated the ring signal output. Line 384 can be coupled by relay contact 382 of relay K1 to either the -48 volt DC output or the +48 volt DC output of the DC supply (not shown). The ring signal on line 90 may be coupled to either the line cards coupled to the A side ring generator or the line cards coupled to the B side ring generator by K2 relay contact 428. These contacts are controlled by cross-over relay circuit K2 578 which receives its input from bus 422. The cross-over relay circuit causes the ring signal for one side of the ring generator card to be crossed over and utilized by the other side of the system if the ring generator on that side fails, as indicated by the A and B CPU valid signals on bus 422.

In the preferred embodiment, the ring generator also includes an alarm sense circuit 428 which senses when the ring voltage generated by the ring generator circuitry is too low and also senses when there has been a failure to generate any ring voltage at all, such as where an open circuit in the transformer occurs. Sensing of an open circuit or other failure to produce a ring signal voltage is accomplished by testing the voltage level on line 90 via line 431 coupling line 90 to an input of alarm sense circuit 428. A low voltage ring signal condition is sensed by line 433 which couples alarm sense winding 435 to an input of alarm sense circuit 428. The other end of winding 434 is coupled to a negative 48-volt DC voltage source (not shown). When the controller desires to test the status of the alarm sense circuit 428, it enables tri-state gate 436 via line 438. Tri-state gate 436 has its input coupled via line 440 to the output of alarm sense circuit 428. The output of tri-state gate 436 is coupled to bit D1 of data main bus 32. The signals on line 440 are not coupled to the output on line 442 unless the enabling signal on line 438 is present. Line 438 is coupled to the output of NAND gate 444 which has its inputs coupled to the card select signal on line 364 and the READ I/O signal on line 446 through inverter 448 and line 450. Thus, tri-state gate 436 is enabled whenever the ring generator card is selected by line 364 and the READ I/O signal goes low, indicating that the data main bus 32 is being read by the controller.

FIGS. 11 and 12 show a logic diagram of the ring generator card 88 in FIG. 1 when placed side by side. Turning now to FIGS. 11 and 12, a more detailed description of the structure of the system disclosed in FIG. 10 will be given. Decoder 374 will be found in the lower middle of FIG. 11. In the preferred embodiment, this chip is a Motorola™ MC 14556 B CMOS dual, binary, one-of-four decoder/demultiplexers. Its enable input is coupled via line 398 to the output of NAND gate 396. This NAND gate has its input coupled via line 394 to the output of inverter 392. The other input of NAND gate 396 is coupled via line 452 to 1K resistor 454. The other end of this 1K resistor is coupled to the card select line 364. Line 364 is also coupled to ground through 1-meg resistor 456. The purpose of the network formed by resistors 456 and 454 is to hold card select line 354 firmly at ground potential during periods of inactivity on that line. The inputs of NAND gate 392 are coupled to 1K resistor 458. The other end of this resistor is coupled to the signal WRITE I/O on line 390. Line 390 is also coupled by resistor 460 to a positive 12-volt DC voltage source. The purpose of the network formed by resistors 460 and 458 is to hold line 390 firmly at a logic one state when the signal WRITE I/O is not being asserted low.

The A input of decoder 374 is coupled by line 460 to 1-kilohm resistor 462. The other end of this resistor is coupled by line 464 to the ADR 1 address line generated by controller 36 in FIG. 1. Line 464 is also coupled to a ground potential by one-megohm resistor 466. The B input of decoder 374 is coupled via line 468 to one-kilohm resistor 470. The other end of resistor 470 is coupled to the ADR 2 line from controller 36 in FIG. 1. This line, 472, is coupled to ground potential by one-megohm resistor 474.

The Q₀ is coupled via line 402 to the clock input of latch chips 476 and 478 of latch 400. In the preferred embodiment, these chips are Motorola™ MC 14175 B quad-type D flip-flops. The D0 input of latch chip 476 is coupled to the output of buffer amplifier 486 by line 478. The D1 input of latch chip 476 is coupled to the output of buffer amplifier 488 by line 368. The D2 and D3 inputs of this latch are coupled to buffer amplifiers 490 and 492 by lines 482 and 484, respectively. The inputs of buffer amplifiers 486 through 492 are coupled to 1-kilohm resistors 494 through 500, respectively. The other sides of these 1-kilohm resistors are coupled to data bits D1 through D4 of data bus 32 from controller 36. Data lines D1 through D4 are respectively coupled to a +12 volt DC potential by 1-megohm resistors 502 through 508.

The purpose of 1-megohm resistors 502-508 and 1-kilohm resistors 494-500 is to form individual networks on each of data lines 1-4 to hold them firmly at a logic one level when there is no activity on the data lines.

The purpose of buffer amplifiers 486-492 is to prevent excessive capacitive loading on data lines D2 and D3 which, if buffer amplifiers 488 and 490 were not present, would be coupled to two inputs, each representing one logic load. The structure of the data bus and controller is such that excessive capacitive loading on the data lines can result in slowness in data transfers or failures of the system to properly function.

Data lines D5 through D8 are similarly connected to data inputs D0-D3 of latch chip 478 utilizing identical 1-megohm and 1-kilohm resistor networks to hold these data lines at a logic one level during periods of inactivity. No buffer amplifiers are present between these resistive networks and chip 478 because none of the data lines D5-D8 is connected to any other load other than inputs of latch chip 478. The Q0 through Q3 outputs of both latch chips 476 and 478 are coupled to the digital data inputs of divide-by-N counter 510 by bus 404.

In the preferred embodiment, divide-by-N counter 510 is a Motorola™ MC 14569 BCP CMOS high-speed programmable divide-by-N, dual, 4-bit BCD/binary counter. The data bits for programming the counter enter the control word input of the chip on bus 404. The clock signal enters the chip on line 512 from the output of buffer 514. The input of buffer 514 is coupled to 1-kilohm resistor 516 by line 518. The other end of 1-kilohm resistor 516 is coupled to an 8-kilohertz clock signal on line 520. Line 520 is also coupled to ground through 1-megohm resistor 522 forming a network with resistor 516 to condition the clock signal for buffer 514.

The purpose of buffer 514 is to give the clock signal on line 520 better definition and amplitude with less noise and to prevent capacitive loading of the clock line 520.

The output of divide-by-N counter 510 is coupled via line 410 to the clock input of flip-flop 412. This flip-flop serves as a square wave generator to transform the pulses on line 410 into a square wave pattern of one-half the frequency line line 414.

Line 414 couples the Q output of flip-flop 412 to one input of NOR gate 528. The Q output of flip-flop 412 is coupled by line 530 to the D input of the same flip-flop, thereby forming a divide-by-two circuit. The set and reset inputs of flip-flop 412 are coupled to ground. In the preferred embodiment, flip-flop 412 is a Motorola™ MC 14013 BCP CMOS, dual, D-type flip-flop. The other input of NOR gage 528 is coupled via line 532 to the output of inverter 534. It is also coupled to one input of NOR gate 536. The other input of this NOR gate is coupled via line 530 to the D input and Q output of flip-flop 412. Together, NOR gates 528 and 536 form power down circuit 416, shown in FIG. 10. The inputs of inverter 534 are coupled to the output of NOR gate 538. This gate has one input coupled to the Q output of latch 422 via line 540, and the other input coupled to the output of NOR gate 542 by line 544.

Latch chip 422 serves as the on/off bit store allowing the controller 36 to power down the ring generator circuit of FIGS. 11 and 12 on command, as explained in the earlier discussion of FIG. 10. The D input of latch chip 422 is coupled via line 482 to the output of amplifier buffer 490 which has its input coupled to data bit D3 of the controller data bus. The clock input of latch chip 422 is coupled via line 420 to the Q1 output of decoder 374. The reset input of the latch chip 422 is grounded. In the preferred embodiment, latch chip 422 is a Motorola™ MC 14013 B CMOS, dual D flip-flop. Controller 36 loads latch chip 422 via the ADR 1 and 2 lines, the card select line 364 and the WRITE I/O signal on line 390. The controller can load a logic 1 or 0 via bit D3 of its data bus thereby setting or resetting the Q output on line 540. This signal ripples through NOR gate 538, inverter 534, and power down NOR gate 536 to ultimately emerge on line 546 coupled to the input of inverter 548. In the preferred embodiment, inverter 548 is a Motorola™ MC 1416 P open collector-type inverter.

Likewise, the signal at the Q output of latch chip 422 on line 540 ripples through NOR gate 538, inverter 534, and NOR gate 528 to emerge on line 550 coupled to the input of open collector inverter 552.

The output of open collector inverter 548 is coupled via line 554 to the negative end of the light emitting diode in optical coupler 558. The positive end of the light emitting diode is coupled through resistor 560 to a positive 12-volt DC supply.

An identical circuit is coupled to the output of open collector inverter 552 for driving the LED of optical coupler 568 in FIG. 12.

From the foregoing, it is seen that by setting or resetting the D3 bit of the data bus, the controller can control these signals at the input of open collector inverters 548 and 552. In one input condition, these inverters will sink current to ground through their outputs. In the other condition, they will refuse to sink current. Thus, the controller has complete control over the current flowing through the light emitting diodes in optical couplers 558 and 568 and thus it has control over the base current of the input transistors of drivers 424 and 426. The lowered light emission from the LEDs causes the transistors in the optical couplers to have a lower emitter current. The controller can therefore shut down current flowing in drivers 424 and 426 to save power.

This shut-down feature is also actuated automatically in the eventuality that both controllers fail. In such a case, the A CPU VALID and B CPU VALID signals on bus 422 in FIG. 12 will assume a logic state which causes the output signal of NOR gate 542 on line 544 to ripple through NOR gate 538, inverter 534, and NOR gates 536 and 528 so as to cause open collector inverters 548 and 552 to shut down the light emitting diodes in the optical couplers. The inputs of NOR gate 542 are coupled to the A CPU VALID and B CPU VALID signals through 10K resistors 570 and 572. Both the A CPU and the B CPU VALID signals are coupled to ground through two megohm resistors 574 and 576. These resistors insure that the A CPU and B CPU valid lines are firmly held at ground potential during periods of inactivity on these lines.

The digital multiplex system disclosed herein utilizes dual ring generators for reliability purposes. Should one ring generator fail, a cross-over circuit will detect the signal indicating that a controller has detected a failure in one of the ring generating cards. This cross-over circuit will cause the still-operating ring generator to switch over to supply a ring signal to the side which has failed. This cross-over circuitry is comprised of inverter 574, open collector inverter 576, and relay K2 578. The inputs of inverter 574 are coupled to the A CPU VALID signal, but, on the other side, the inputs would be coupled to the B CPU VALID signal. The output of inverter 574 is coupled to the input of open collector inverter 576 which has its output coupled to the coil of relay K2 578. The other end of the coil of K2 is coupled to the positive 12-volt DC supply. When a logic one appears at the input of open circuit inverter 576, the inverter output goes to logic zero and sinks current through the relay coil of K2, thereby energizing the relay. This causes relay contacts 428 in the upper-center portion of FIG. 12 to operate so as to connect the ring signal on line 90 to the cross-over line 582 coupled to the other ring generator as opposed to the normal ring signal line 90.

Latch 370 in FIG. 11 is the plus or minus polarity bit memory. Its data input is coupled by line 368 to the output of buffer amplifier 488 which in turn is coupled to the D2 bit of data main bus 32. The clock input of this chip is coupled via line 376 to the Q2 output of decoder 374. The set and reset inputs of latch 370 are grounded. In the preferred embodiment, this latch is a Motorola™ MC 14013 BCP. Controller 36 in FIG. 1 can control the polarity of the DC voltage applied to the DC input 384 of secondary winding 386 of the transformer 388 in FIG. 12. Controller 36 exercises this control by setting or resetting bit D2 of its data bus after addressing latch 370 via the ADR 1 and 2 lines and the ring generator card select line 364 in address and control bus 46. The Q output of latch 370 is coupled to the input of open circuit inverter 378 by line 376. The output of this inverter is coupled by line 380 to the coil of relay K1 372. The other end of this coil is connected to a positive 12-volt DC supply. Thus, controller 36 can control the energization state of relay K1 through bit D2 of its data bus, latch 370, and inverter 378. Relay contacts K1, 382 in the upper-mid-portion of FIG. 12, then connect either the -48 volt supply, coupled to line 582, or the +48 volt DC supply, coupled to line 584, to the DC input 384 of secondary winding 386. This causes either a positive or negative 48-volt DC level to appear on line 90. The square wave ring signal is superimposed upon this DC level by the operation of optical couplers 418 driving push-pull drivers 424 and 426 to drive current alternately through primary windings 432 and 434 to transformer 388 in FIG. 12.

The operation of Darlington pair 424 is conventional and apparent to those skilled in the art. Likewise, for darlington pair 426. Capacitors 584 and 586 function in a noise suppression capacity.

The transistor in optical coupler 568 has its base coupled to a 1-megohm resistor 588, the other end of which is coupled to a -48 volt DC supply. The collector of this transistor is grounded, and the emitter is coupled to an 8.2-kilohm resistor 590. The other end of this resistor is coupled to the base of transistor 592. The base of transistor 592 is also coupled by a 10-kilohm resistor 594 to the emitter of transistor 592. The emitter of transistor 592 is cuopled to the base of transistor 596. The collector of transistor 592 is coupled to the collector of transistor 596 and to the negative end of diode 598 and to one end of primary winding 432 by line 600. The base of transistor 596 is coupled by 150-ohm resistor 602 to the emitter of transistor 596. The emitter of transistor 596 is coupled to the positive end of diode 598 and to the ungrounded end of capacitor 584 by line 604. Line 604 is also coupled to the emitter of transistor 606. The emitter of transistor 606 is coupled to the base of transistor 606 by 10K resistor 610. The base of transistor 606 is also coupled through 8.2K resistor 612 to the emitter of the transistor in optical coupler 558 by line 614. The base of the transistor in optical coupler 558 is coupled through 1-megohm resistor 616 to a -48 volt DC supply. The collector of the transistor in optical coupler 558 is grounded.

Returning to Darlington pair 426, the emitter of transmitter 606 is also coupled to the emitter of transistor 608 by 150-ohm resistor 618. The emitter of transistor 606 is also coupled to the base of transistor 608 by line 620. The collector of transistor 608 is coupled to the collector of transistor 606 and to the negative end of diode 622 and to one end of primary winding 434 by line 624. The emitter of transistor 608 is also coupled via 10-ohm resistor 626 to the ungrounded end of capacitor 586. The two ends of primary windings 432 and 434, not already mentioned, are coupled to ground by line 628. The ends of capacitors 584 and 586 not already mentioned are coupled to ground via line 628.

The -48 volt DC supply on line 582 is coupled by 10-ohm resistor 628 to 0.047-microfarad capacitor 630. Relay contacts 382 are bypassed by 0.047-microfarad capacitor 630 and 20-ohm resistor 632 for spark suppression purposes. The positive 48-volt DC supply coupled to line 584 is also coupled to 0.047-microfarad capacitor 634 and 20-ohm resistor 636 which bypass the second set of ganged contacts 382 for spark suppression purposes.

The 20-ohm resistor 638 and 1-microfarad capacitor 640 coupled from DC input 384 to line 431 serve to shunt off any high voltage peaks which occur in secondary winding 386 due to switching action by the relay contact. This is necessary to prevent the peaks from destroying any CMOS logic.

Line 431 coupling the output of secondary winding 386 to the input of alarm sensor 428 provides the information necessary for the alarm sensor 428 to determine whether there has been an open circuit in the transformer or a failure to generate any ring signal voltage on line 90. Line 431 is coupled to a 47K resistor 642 which has its opposite end coupled to the negative end of a light emitting diode in optical coupler 644. The positive end of the light emitting diode is grounded. Diode 646 bypasses the LED for protection purposes having its positive end coupled to the negative end of the LED and vice versa. The base of the transistor in optical coupler 644 is left open. The collector of this transistor is coupled to the 12-volt supply by 51K resistor 650. The collector is also coupled to the collector of transistor 652 and the input of tri-state gate 654 by line 656. The emitter of the transistor in optical coupler 644 is coupled to the base of transistor 652 by line 658. The emitter of transistor 652 is coupled to ground through line 662. The output of tri-state gate 654 is coupled to bit D2 of data main bus 32 by line 664.

Line 433 couples secondary winding 435 of transformer 388 to the positive end of diode 666. The other end of secondary winding 435 is coupled to the -48 volt DC supply. The negative end of diode 666 is coupled through 10K resistor 668 to the positive end of electrolitic 10-microfarad capacitor 670. The other end of this capacitor is coupled to the negative 48-volt supply. Diode 666, resistor 668, and capacitor 670 form a detector circuit which detects the signal voltage appearing on secondary winding 435. Thus, the output voltage at node 672 is indicative of the amplitude of the ring signal waveform generated by drivers 424 and 426. This signal voltage is coupled by line 674 to the negative input of comparator 676. Node 672 is also coupled to the 48-volt negative DC supply by 100K resistor 678. The positive input of comparator 676 is coupled via line 680 to 330K resistor 682. Line 680 is also coupled to the negative 48-volt supply through 9.53K resistor 684. Line 680 is also coupled to 16.9K resistor 686, the other end of which is coupled to one end of 0.1-microfarad capacitor 688. The other end of this capacitor is coupled to the negative 48-volt DC supply. Line 690 couples the node between resistor 686 and the first end of capacitor 688, and the positive end of zener diode 692 to ground through 2.7K resistor 694. The other end of zener diode 692 is coupled to the -48 volt supply. Line 690 is also coupled to one end of a 0.1-microfarad capacitor 696 and to pin 7 of comparator 676. In the preferred embodiment, comparator 676 is a generic 741 type manufactured by several sources. The other end of capacitor 696 is coupled to the -48 volt DC supply and to pin 4 of comparator 676. The output of comparator 676 is coupled to 2.7K resistor 698 and to the other end of 330K resistor 682 via line 700. The other end of resistor 698 is coupled to the positive end of the light emitting diode in optical coupler 702. The negative end of this light emitting diode is coupled to the negative 48-volt DC supply. The transistor of this optical coupler 702 has its collector coupled to the positive 12-volt DC supply and its emitter coupled via line 440 to the input of tri-state gate 436. The emitter of the transistor is also coupled through a 100K resistor 706 to ground.

When the controller wishes to read alarm bit A, it selects the ring generator card using the CARD SELECT signal on line 364 and enters a read cycle. This causes the READ I/O on line 446 to go low. This in turn causes inverter 448 to raise its output on line 450 which causes NAND gate 444 to lower its output line 438 thereby enabling tri-state gate 436 and coupling line 440 to bit D1 of the controller data bus via line 442. The voltage on node 672 is compared by comparator 676 against a reference voltage established on line 680 by the resistor network coupled thereto. If the voltage peaks on node 672 are reaching above the reference voltage, comparator 676 outputs signals on line 700 which are optically coupled by coupler 702 to the input of tri-state gate 436.

The operation of optical coupler 644 in FIG. 12 and transistor 652 in conjunction with alarm bit B will be apparent to those skilled in the art.

Description of the Re-frame FIFO Buffer in the Multiplexer

FIGS. 13A and 13B show a logic diagram of the re-frame buffer 178. Turning now to FIGS. 13A and 13B, a more detailed explanation of the operation of re-frame buffer 178 in FIG. 5 will be given. Line receiver chip 800 in FIG. 13B receives the NRZ, i.e., unipolar, data on line 58, from span line terminating card 54 in FIG. 1. Line receiver chip 800 also receives a clock input signal on line 62 also from span terminating card 54 in FIG. 1. This clock signal has a frequency of 1.544 megahertz, and is synchronized with the occurrence of each bit in the serial bit stream on line 58. In the preferred embodiment, line receiver chip 800 is a Rockwell™ R8060.

The line receiver chip converts the serial data at its input to parallel output data on lines CDB 1-8. It also generates a channel clock signal on line 802 which marks each time slot. That is, the channel clock signal on line 802 goes high once for each time slot. Line 802 is coupled to the input of inverter 804 which has its output coupled via line 182 to the CP or count input of counter chip 806. The signals on line 182 serve to increment counter 806 such that the output data on bus 176 indicate the binary number representing the number of the time slot which is currently being received by line receiver 800.

Line receiver 800 also generates a frame alarm signal on line 812 which goes high when there is a loss of frame synchronization with the input data. That is, when line receiver 800 fails a predetermined number of times to detect the frame bit in the input data at the end of all twenty-four time slots in a frame, it will set the frame alarm signal on line 812. Line receiver 800 also generates a channel sync signal on line 814 and a write inhibit signal on line 816. The channel sync signal is a low-going pulse which occurs at the end of each frame. The write inhibit signal indicates when it is impermissible to write new data to the input of line receiver 800. This signal goes high for a portion of each time slot. The write inhibit signal on line 816 is coupled to the input of AND gate 818, the other input of which is coupled via line 820 to the output of inverter 822. This inverter has its input coupled to the channel sync signal on line 814. The output of AND gate 818 is coupled to the reset input of counter 806 by line 824. The output of AND gate 818 on line 824 goes high at the end of every frame for a portion of a time slot. This resets time slot counter 806 so that it may begin counting the time slots of the next frame.

Line receiver chip 800 also generates a carrier loss alarm on line 177 which is coupled to controller 36 in FIG. 1 through the status register in the multiplexer, as shown in FIG. 5, to indicate that no serial data is being received from the span line.

The outputs of time slot counter 806 on bus 176 are coupled to the data inputs of FIFO buffer chips 828 and 830. The data outputs of line receiver chip 800 are coupled to the data inputs of FIFO buffer chips 832 and 834 by bus 176. Another layer of FIFO buffer memory is provided by FIFO buffer chips 836, 838, 840 and 842.

FIFO buffer chips 828 through 842 are RCA™ 40105B chips in the preferred embodiment. These chips work as follows. Data presented at data inputs D0-D3 is loaded into the first level of the stack when the shift input signal, marked SI, goes high. Each time the shift input signal goes high, the data present at a D0, D1, D2, and D3 input is loaded into the next level down in the stack. The DIR output signal indicates by one binary state that the FIFO memory chip is ready to accept new data. The other binary state indicates that the FIFO memory chip is not ready to store new data. The master reset input, marked MR, clears the entire stack when it is driven to a high level.

Each of FIFO memory chips 828-842 contains a stack of sixteen 4-bit words. The outputs of the first level of the stack from FIFO chips 828, 830, 832, and 834 on bus 844 are coupled to the input bits D0, D1, D2, and D3 of the second level of the FIFO stack comprised of chips 836, 838, 840, and 842. Therefore, the overall re-frame FIFO buffer is thirty-teo levels deep and sixteen bits wide. This comprises approximately a frame-and-a-half of serial input data.

When the DOR signal of each FIFO buffer chip is high, data is present and ready to be read from outputs Q0 through Q3 so long as the tri-state input, marked TS, is held low. It should be noted for FIFO chips 828, 830, 832, 834, 836 and 838 that the tri-state input is perpetually grounded. Therefore, these chips can be read as soon as the data output ready signal, DOR, goes high.

The shift-out input, marked SO, causes, when it is driven high, the next level down in the stack to be moved to the output bits Q0-Q3.

The purpose of the FIFO stack is to hold the 8-bit parallel digital data for each time slot together with its time slot count until this time slot count comes up again on the internal data but of the multiplexer. The re-frame buffer then releases it to the internal PCM data bus 180 for transfer to various locations in the multiplexer. This is accomplished as follows. During time slot zero, the serial data coming into line receiver chip 800 is converted to parallel data on lines CDB1 through CDB8 and coupled to the data inputs of FIFO chips 832 and 834 by bus 176. At the same time, the binary information on output bits Q1-Q5 of time slot counter 806 representing time slot zero is coupled via bus 176 to the data input of FIFO buffers 828 and 830. This data is loaded into the first level of the stack and appears on data outputs Q0 through Q3 of FIFO buffer chips 828, 830, 832, and 834. The appearance of this data at the outputs is marked by the transition of the DOR signal of FIFO buffers 828-834 from a low to a high level. These transitions are coupled to the shift in input pins of FIFO buffers 836, 838, 840, and 842 by lines 845, 846, 848, and 850, respectively. This causes the data at the outputs of buffer chips 828, 830, 832, and 834 to be loaded into the first level of the stack comprised of buffers 836-842 via bus 844. This data is then presented at outputs Q0-Q3 of buffer chips 836, 838, 840, and 842 which appearance is marked by a low to high transition of the DOR outputs of these buffer chips.

As soon as buffer chips 836-842 are ready to receive another data word, the DIR signal output goes high. These transitions are coupled to the shift out inputs of buffers 828-834 via lines 852, 854, 856, and 858, respectively. This low to high transition causes buffers 828 through 834 to shift the next level down in the stack to their outputs. Prior to that time, the next level down in the stack comprised of buffers 828-834 had been filled by the parallel data of time slot one. This data was loaded at the end of time slot one when the channel clock signal on line 802 from line receiver 800 changed state so as to indicate that the data on lines CDB 1-8 was then the data for time slot one. This change of state of the channel clock signal was coupled through inverter 804 to time slot counter 806 incrementing it to indicate that time slot one was then being received. This change of state on line 182 was also coupled to the shift-in input, SI, of buffer chips 828-834. This caused these chips to load the data at their inputs into the next empty level of the stack. This data was then transferred to the outputs of buffers 836-842 in a similar fashion as was the data for time slot zero.

As the time slot count for the multiplexer internal but 180 matches the time slot count stored in FIFO buffers 836 and 838, the data residing at the top of the FIFO stacks in buffers 840 and 842 is loaded onto internal PCM data bus 180. Therefore, the FIFO stack acts as an elastic buffer to temporarily store incoming data until its time slot, as determined by the internal time slot counter in control timing 184 in FIG. 5, comes up whereupon it is transferred via the internal PCM data bus to wherever it happens to be going as determined by controller 36. In this fashion, any variable delays in transmission over the span line will not cause multiplexer 42 to get out of synchronization with the multiplexer in the terminal at the other end of the span line.

More specifically, loading of the data from FIFO buffers 840 and 842 onto internal PCM data bus 180 is accomplished by comparing the stored time slot count at the top of the FIFO stack in chips 836 and 838 with the time slot count as determined by control timing 184 in FIG. 5. This comparison is done by comparator 860. The B side of this comparator, inputs B0, B1, B2, and B3, are coupled via bus 210 to the outputs of the time slot counter in control timing 184, i.e., bits TC4, TC8, TC16, and TC32. Resistors 862, 864, 866, and 868 are all 10K resistors and couple these bits to a positive level supply to hold them affirmatively at a logic one level unless they are being asserted at a logic zero level by the time slot counter. In a preferred embodiment, comparator 860 is a Motorola™ MC 14585 BCP.

The four outputs from FIFO buffer 838 are coupled to the A side inputs of comparator 860, bits A0-A3, by bus 870. The A Greater Than B and VDD inputs are coupled to the positive 5-volt supply by line 872. The A Less Than B and VSS inputs are coupled to ground by line 874. The A Equals B input is coupled to the output of AND gate 876 by line 878. In this configuration, when line 878 is a logic one, the A equals B output on line 880 will be a logic one if A0 equals B0, A1 equals B1, A2 equals B2, and A3 equals B3.

Line 880 is coupled to one input of exclusive-OR gate 882 which has its other input coupled by line 884 to the +5 volts supply. In this configuration, exclusive-OR gate 882 is functioning as an inverter and, indeed, a simple inverter would serve the purpose. The output of inverter 882 is coupled via line 884 to the tri-state inputs, TS, of FIFO buffers 840 and 842.

Comparator 860 is enabled through the following network. One input of AND gate 876 is coupled by line 886 to the output of exclusive-OR gate 888. One input of this exclusive-OR gate is coupled by line 890 to the TC2 bit from the time slot counter in control timing 184 of FIG. 5. Line 890 is also coupled via 10K resistor 892 to the +5 volts supply to hold this line firmly at a logic one when TC2 is not being affirmatively asserted low. The other input of exclusive-OR gate 888 is coupled via line 894 to the output of inverter 896. This inverter has its input coupled via line 898 to the Q3 output of FIFO buffer 836, The other input of AND gate 876 is coupled via line 900 to the output of AND gate 902. This AND gate has its inputs coupled to the data output ready outputs, DOR, of FIFO buffers 836, 838, 840, and 842 via bus 904. Therefore, if any of buffers 836-842 do not have data ready on their outputs, one of the lines in bus 904 will be at a logic zero level. This will cause line 900 to be at a logic zero level. Thus, line 900 will be a logic one only if all four FIFO buffers 836-842 have data ready at their output.

Exclusive-OR gate 888 in the configuration shown in FIG. 13A acts as a two-bit comparator comparing the Q3 output bit of FIFO buffer 836 to the TC2 output bit from the time slot counter in control timing 184 of FIG. 5. The Q3 bit of FIFO buffer 836 holds the data which was loaded into the D3 input of FIFO buffer 828 from the Q1 output of time slot counter 806 via bus 176. By inverting the Q3 bit in inverter 896, exclusive-OR gate 888 generates a logic one on its output line 886 when the signals on line 890 and Q3 from FIFO buffer 836 are at the same logic level. When line 886 is high and line 900 indicates that all of buffers 836 through 842 have data ready at their outputs, line 878 is raised to a logic one level, thereby enabling comparator 860 which makes the comparison. If the time slot counter bits TC4, TC8, TC16, and TC32 are the same as the output bits Q0-Q3 of FIFO buffer 838, a logic one appears at the A Equals B output on line 880. This is inverted in inverter 882 and coupled via line 884 to the tri-state inputs of FIFO buffers 840 and 842. The logic zero on line 884 enables FIFO buffers 840 and 842 to drive the internal PCM data bus 180.

The shift-out inputs of FIFO buffers 836 through 842 are coupled via line 902 to the output of inverter 904. This inverter has its input coupled to the output of AND gate 906 by line 908. This AND gate has one input coupled to the dump FIFO signal on line 910 and the other input coupled via line 912 to the Q output of latch 914. Line 910 is also coupled via 10K resistor 916 to the positive five-volt supply such that line 910 will be affirmatively held at logic one when the dump FIFO signal is not being asserted low.

The reset input of latch 914 is grounded and the clock input is coupled to the LOAD RB signal on line 918. The D input of latch 914 is coupled via line 880 to the A Equals B output of comparator 860. Latch 914 also has its set input and pin 7 coupled to ground. In the preferred embodiment, this latch is a generic type 4013B flip-flop. Likewise, time slot counter 806 is a generic 4024B.

The signal LOAD RB is a positive going pulse that occurs during every other write cycle of the signal R/W generated by control timing 184 in FIG. 5. It should be noted at this point that control timing 184 is coupled as needed to all the blocks shown in FIG. 5 with various signals from the PCM bus clock and timing bus 146. The signal R/W controls the read and write cycle for each MUX internal time slot on internal PCM data bus 180.

The signal DMP FIFO is a positive going pulse during every other read cycle of the signal R/W from control timing 184. Thus, the signal on line 910 is a logic one during every other read cycle on the multiplexer and the signal on line 918 is a logic one during every other write cycle of the multiplexer. The circuitry for generating the signals dump FIFO and load RB is not shown in FIG. 13 but will be apparent to those skilled in the art.

Because multiplexers 42 and 44 each have two internal time slots for every one time slot on the span line, and because each internal multiplexer time slot is comprised of one read cycle and one write cycle, the signal LOAD RB on line 918 will load latch 914 only once during every time slot of the span line. Because its data input is coupled via line 880 to the A Equals B output of comparator 860, latch 914 will load a logic one only when the time slot of the span currently being processed by the multiplexers, as indicated by the TC2, 4, 8, 16, and 32 bits on the B input of comparator 860 equals the time slot count indicated at the output of FIFO buffers 836 and 838. When latch 914 loads a logic one, its Q output on line 912 will go to a logic one level. Thus, line 908 at the output of AND gate 906 will go to a logic one level only when the time slot counts are equal and the read cycle of an internal MUX time slot is occurring. The signal on line 908 is inverted in inverter 904 and applied via line 902 to the shift-out inputs of buffers 836 through 842. This will cause the next words down in the stack to be shifted to the outputs and held there. The data previously on these outputs will have been read from the internal PCM data bus 180 during the previous internal MUX read cycle within the same time slot on the span line.

The master reset of FIFO buffers 828 through 842 are all coupled via line 922 to the Q output of latch 924. The clock input of latch 924 is coupled via line 182 to the output of inverter 804 which has its input coupled to the channel clock signal, CHCLK, on line 802. The data input of latch 924 is coupled via line 926 to the output of AND gate 928. This AND gate has one input coupled to the signal CARR. LOSS, and the other input coupled via line 930 to the output of AND gate 932. The inputs of AND gate 932 are coupled to the data input ready outputs of FIFO buffers 828 through 834 via bus 934. The output of AND gate 932 on line 930 thus will be a one only when all of FIFO buffers 828-834 are ready to receive additional data at their data inputs D0-D3. Assuming that no loss of carrier is indicated by the signal CALRM on line 177 from line receiver 800, the signal CARR. LOSS on line 936 will always be a logic one. Therefore, as long as the signals on bus 934 all indicate readiness to receive additional data at the end of each time slot, as marked by the signal on line 182, latch 924 will continue to load logic ones via line 926. However, as soon as any one of FIFO buffer chips 828-834 fills up all sixteen of its 4-bit words, one of the signals on bus 934 will go to a logic zero level and be in that state at the end of a time slot. When this occurs, line 930 goes to a logic zero level causing line 926 to go to a logic zero level, thereby loading a zero into latch 924 at the end of one time slot. This sets the Q output on line 922 at a logic one level which signals the master resets of FIFO buffer chips 828-842 to clear the entire contents of the FIFO buffer. The logic one level on line 922 also constitutes the signal FRAME LOSS which is coupled to the controllers indicating that a frame of data has been lost due to overflow of the FIFO buffer.

There is disclosed below the programs for the controllers in the system. Appendix I is the program for the remote terminal controllers and Appendix II is the program for the central office terminal controllers. These programs are not critical to the invention, and any other programs which accomplish the same functions are intended to be included within the scope of the claims appended hereto. Likewise, any hardware implementations of the circuitry disclosed herein which accomplishes the functions as disclosed herein is also intended to be included within the scope of the claims. ##SPC1## ##SPC2## ##SPC3## ##SPC4## 

What is claimed is:
 1. A maintenance circuit for a digital multiplexed communication system having a central office terminal and a remote terminal with a plurality of digital-to-analog and analog-to-digital line card interface circuits including audio line drivers and receivers for individual telephone lines, said maintenance circuit located in said remote terminal to automatically test said line card interface circuits whenever they are idle, comprising:a data bus coupled to said line card interface circuits for carrying digital data; a generator means coupled to said data bus for generating a plurality of words of digital test data; a detector means coupled to said data bus for detecting test data and for generating an output signal; and means to automatically, continuously determine which said line card interface circuits are idle and for causing said generator means to send said digital test data to said idle line card interface circuits and for causing said detector means to be coupled to said idle line card interface circuits to detect the presence of digital test data after it has been converted to analog form and then back to digital form by said idle line card interface circuits.
 2. Apparatus in a terminal of a telephone system for testing a voice channel within said terminal of said telephone system carrying plural audio channels, each said channel having a line card interface circuit coupling said terminal to a telephone line each said line card interface circuit including a digital-to-analog converter for carrying data toward said telephone line, and an analog-to-digital converter for carrying data toward said terminal and audio circuitry for the analog side of said interface circuit, comprising:means for periodically, automatically monitoring each of said plural audio channels to select an audio channel which is not in use; means for generating a test signal and for connecting said test signal to said line card interface circuit so that said test signal propagates through said digital-to-analog converter, said audio circuitry and said analog-to-digital converter of said audio channel which is not in use; and means for detecting whether said line card interface circuit which is not in use is operative by detecting the presence of said test signal at the output of said analog-to-digital converter.
 3. An apparatus, as defined in claim 2, further comprising means responsive to said detecting means for indicating when a failure has occurred.
 4. An apparatus, as defined in claim 2, further comprising means responsive to said detecting means for indicating when and on which audio channel a failure has occurred.
 5. An apparatus, as defined in claim 2, wherein said telephone system includes a digital time division multiplexer and said generator means generates a plurality of digital words which, in combination, describe an analog test signal.
 6. An apparatus, as defined in claim 5, wherein said plurality of digital words are directed by said monitoring means to said selected audio channel through said multiplexer and said plurality of digital words are looped through said selected audio channel and back through said multiplexer to said detector means.
 7. An apparatus, as defined in claims 5 or 6, wherein all of said plural audio channels are connected to said multiplexer by one bus.
 8. A self-diagnosing telephone terminal for use in a digital multiplexed span line telephone system for coupling a plurality of telephone lines to said span line, comprising:a digital-to-analog converter for coupling digital signals from said span line to a telephone; an analog-to-digital converter for coupling analog signals from said telephone to said span line; means for sensing whether said telephone is on hook; audio signal processing means for driving analog signals from said digital-to-analog converter onto said telephone lines and for receiving analog signals from said telephone lines and for driving them into the input of said analog-to-digital converter; means responsive to said sensing means for periodically, automatically injecting a digital test signal into said digital-to-analog converter when said telephone is on hook; means for looping output signals from said digital-to-analog converter to the input of said analog-to-digital converter through said audio signal processing means; and means responsive to said sensing means for detecting said digital test signals from the output of said analog-to-digital converter.
 9. An apparatus, as defined in claim 8, further comprising a means responsive to said detecting means for indicating when said digital test signals are not present at the output of said analog-to-digital converter.
 10. An apparatus, as defined in claim 8, further comprising a time division multiplexer means coupled between said digital-to-analog and analog-to-digital converters and said span line by a data bus and connected to said means for sensing and to said means for injecting, said multiplexer means for transferring said digital test signal from said means for injecting to the digital-to-analog converter coupled to said on-hook telephone selected by said means for sensing and for transferring said digital test signals from said analog-to-digital converter back to said means for detecting said transfers under the automatic control of said means for sensing.
 11. Apparatus for testing an audio channel within a terminal of a telephone system in which plural audio channels share a common span line, where said system includes at least two terminals connected by said common span line, comprising:plural line termination means including per line audio amplifier circuits for connecting said terminal with plural telephone lines, said line termination means superimposing said plural audio channels of said plural telephone lines on said common span line; a maintenance circuit comprising:means for sensing which of said plural telephone lines is not in use, for automatically generating a test signal, and for coupling said test signal to line termination means of said telephone line which is not in use; means in said line termination means for looping said test signal back to said maintenance circuit through said per line audio amplifier circuits; and means in said maintenance circuit for detecting said test signal.
 12. An apparatus, as defined in claim 11, further comprising means responsive to said detecting means for ascertaining if said line termination means is operative and indicating the status of said termination means.
 13. An apparatus, as defined in claim 11, wherein said line termination means includes a time division multiplexer.
 14. An apparatus, as defined in claim 11, wherein said telephone system is a digital time divisionmultiplexed system and wherein said line termination means converts analog signals from said plural audio channels into digital signals and multiplexes them through a time division multiplexer onto said common span line and demultiplexes digital signals from said span line and converts them to audio signals and couples them to selected ones of said audio channels and wherein said means for sensing generates a digital test signal and causes said line termination means to couple said digital test signal to the idle telephone lines automatically, periodically selected by said means for sensing.
 15. Apparatus for testing a voice channel within a terminal of a multiplexed digital telephone system, where said system includes at least two terminals connected by a span line, comprising:plural line termination circuits for connecting said terminal with plural telephone lines, said circuits converting between analog signals on said telephone lines and digital signals in said terminal and including per line audio amplifiers to drive said telephone lines with analog signals and receive analog signals from said telephone lines; means for multiplexing between digital signals in said plural line termination circuits and said span line to provide simultaneous transmission on said span line for a selected group of said plural telephone lines; test circuit comprising:means for sensing which of said plural telephone lines is not in use, for automatically generating a digital test signal, and for coupling said test signal through said multiplexing means to the line termination circuit of said telephone line which is not in use; means in said line termination circuits for looping said test signal through said per line audio amplifiers and back through said multiplexing means to said test circuit; and means in said test circuit for detecting said test signal.
 16. An apparatus, as defined in claim 15, further comprising an alarm means responsive to said detector means for indicating when no test signal has been detected for a selected telephone line not in use.
 17. An apparatus for self-testing a digital multiplexed communication system, comprising:a data bus for carrying digital data describing an analog waveform; a plurality of line interface means coupled to said data bus, said plurality of line interface means including a plurality of per line A/D and D/A conversion circuits and a plurality of audio line driver and line receiver amplifiers coupling a plurality of telephone lines to said data bus; a test signal generator coupled to said data bus, generating a plurality of digital test words, and placing said digital test words on said data bus; a detector coupled to said data bus, detecting said digital test words on said data bus; a multiplexer coupled to said data bus, transferring said digital test words from said test signal generator to a selected one of said plural line interface means, and transferring said digital test words from said selected one of said line interface means to said detector on said data bus during selected time slots; and a controller coupled to said test signal generator, said detector, said multiplexer and said plural line interface means, said controller for selecting the line interface means for said transfer, and to cause said multiplexer to transfer said digital test words from said test signal generator to a selected line interface means where said digital test signals propagate through said per line conversion and audio amplifier circuits and to transfer said digital test signal from said selected line interface means to said detector.
 18. An apparatus, as defined in claim 17, wherein said plurality of line interface means includes means for converting said digital test words to an analog signal, looping said analog signal through the audio channel of said line interface means and reconverting said analog test signal to digital words for transfer back to said detector.
 19. A maintenance circuit for a digital multiplexed communication system coupling a plurality of analog telephone lines coupled to telephones to a digital data transmission line, comprising:line interface means having a digital-to-analog converter and an analog-to-digital converter and audio interface circuits for each telephone line to interface said analog telephone lines to said data bus and for indicating when said telephones coupled to said telephone lines are on-hook and not in use; a PCM data bus for carrying digital representations of analog waveforms; a toner generator coupled to said PCM data bus for generating a plurality of words of digital test data which, in combination, describe an analog test signal and for placing said plurality of digital words on said data bus; a detector coupled to said PCM data bus for detecting said test signal and for generating an output signal when said test signal is present; an alarm circuit coupled to said main data bus; a multiplexer having a line receiver coupled to a serial data input for converting serial format input data on said digital data transmission line to parallel format data for outputting on said PCM data bus and having a line transmitter coupled to a serial data output for converting parallel format data from said PCM data bus to serial format data for outputting on said digital data transmission line, said multiplexer coupled to said line interface means, said tone generator, and said detector by said PCM data bus, and having a control word input coupled to said main data bus for receiving a digital control word, said multiplexer for establishing a plurality of time slots and for establishing a data link for each time slot via said PCM data bus between one of said line interface means and said line receiver and said line transmitter or for establishing a data link between said detector and said tone generator and one of said line interface means, the selection of circuits to be linked and assignment of time slots controlled by said digital control word data; and a controller coupled to said main data bus, and having address and control lines coupled to said multiplexer, said controller for generating a control word for each time slot established by said multiplexer, said controller also coupled to said line interface means by said data, address and control lines, said controller programmed to test each line interface means to determine which telephones are on-hook and not in use and to generate a control word for a self-test time slot, said control word causing said multiplexer to transfer said digital test data from said tone generator to said line interface means for any telephone line not in use and to cause said multiplexer to transfer digital data from said line interface means, which data has been converted to an analog test signal, propagated through said audio interface circuits and reconverted to digital test data, to said detector during the assigned time slot, said main data bus, address and control lines of said controller also coupled to said detector for controlling when said detector is enabled, and said controller programmed to read said output signal from said detector to determine when said line interface circuitry is functioning properly, and to cause said alarm circuit to indicate a failure on the tested line when no test signal is detected and said controller programmed to continuously so test idle line interface means.
 20. An apparatus, as defined in claim 19, wherein said detector includes:a time slot counter coupled to said clock and timing bus from said multiplexer for counting the number of the time slot which said multiplexer is in at any time and for generating a binary representation thereof; a time slot register coupled to said control, address and data lines from said controller for storing the binary representation of the time slot number assigned by said controller for self-testing of idle line interface circuits; a comparator coupled to the binary outputs of said time slot counter and said time slot register for comparing the binary numbers appearing at each output and for generating an output signal indicating when there is a match; a from/to control circuit coupled to the output signal of the comparator and coupled to a data line from the output of the time slot register, said data line carrying a digital data bit which is set or cleared by a data line from the controller, and said from/to control circuit coupled to the clock and timing bus from the multiplexer, said from/to control circuit for controlling whether the detector loads data from the data bus when data is being transferred from the tone generator a line interface circuit or when data is being transferred to the detector from a line interface circuit and for generating an output signal so indicating; and a first means coupled to the data bus and to the output signal of the from/to control circuit for loading data upon command from the from/to control circuit from the data bus and detecting whether it describes an analog signal.
 21. An apparatus, as defined in claim 20, wherein said time slot counter is comprised of:a NAND gate having an output and having its inputs coupled to the proper timing and control signals from the multiplexer such that said output changes state once for every time slot in which the multiplexer makes a data transfer; a counter having its count input coupled to the output of said NAND gate and having its reset input coupled to the proper timing and control signal from the multiplexer such that the counter is incremented once for every transition at the output of the NAND gate but is reset to its starting count after a predetermined number of time slots; and a latch having its data input coupled to the output of said counter and having its reset and clock inputs coupled to the proper timing and control signals from the multiplexer such that the latch is loaded from the counter once per time slot and reset after a predetermined number of time slots have passed.
 22. An apparatus, as defined in claim 20, wherein said from/to control circuit includes:a NAND gate having an output for carrying said from/to signal and having a first input coupled to the output of said comparator and having a second input and having a third input coupled to the control signal of said multiplexer indicating when said multiplexer is loading said first data bus; an inverter having its output coupled to said second input of said NAND gate; and an exclusive-OR gate having its output coupled to the input of said inverter and having a first input coupled to said from/to bit at the output of said register, and having a second input coupled to a control signal from said multiplexer indicating when said multiplexer is writing to or reading from said first data bus.
 23. An apparatus, as defined in claim 20, wherein said first means includes:a flip-flop having its set and reset inputs grounded and having its data input coupled to a bit on said first data bus and having its clock input coupled to the output signal of said NAND gate such that a bit from the first data bus is loaded upon a predetermined transition of said output signal; a first resistor coupled to the output of the flip-flop; a second resistor coupling ground to the opposite end of the first resistor thereby defining a node; a first capacitor coupled to said node; a second capacitor coupled to said node; a third resistor coupled to the opposite ends of said first and second capacitors defining third and fourth nodes; a first differential amplifier having its negative input coupled to said third node and its output coupled to said fourth node, thereby defining an active filter; a fixed voltage reference source coupled to the positive input of said first differential amplifier; a buffer amplifier coupled to the output of said first differential amplifier and having an output; a third capacitor coupled between the output of the buffer amplifier and ground; and a second differential amplifier having its positive input coupled to the output of said buffer amplifier and having its negative input coupled to the fixed voltage reference such that a peak detector is formed, said second differential amplifier for generating an output signal indicating the presence of transitions of the bit of said first data bus said flip-flop is coupled to when said fixed reference voltage is set at a predetermined value.
 24. An apparatus, as defined in claim 19, wherein said line interface means comprises:a buffer coupled to the said data bus and to said address bus and to said clock and timing bus for storing data to be transferred to the multiplexer and for storing data transferred from the multiplexer; a digital-to-analog converter coupled to said buffer and to said clock and timing bus for converting the binary data in the buffer to an analog signal; a receive filter coupled to the output of digital-to-analog converter for removing any quantization noise from the analog signal; an amplifier coupled to the output of the receiver filter for amplifying the analog signal; a transformer having a first winding coupled between the output of the amplifier and ground; a second winding in said transformer; an amplifier having its input coupled to said second winding for amplifying signals inductively coupled onto said second winding; a transmit filter coupled to the output of amplifier for removing frequencies higher than one-half of the sampling frequency; and an analog-to-digital converter coupled to the buffer and to the clock and timing bus and coupled to the output of transmit filter for converting the analog signal at the output of the transmit filter to digital data describing an analog signal and for loading said buffer upon a control signal from said multiplexer.
 25. An apparatus, as defined in claim 19, further comprising:a digital data transmission line termination circuit coupled to said serial data input and serial data output of said multiplexer to convert the serial format data to digital data into a bipolar code suitable for transmission on a T1-type digital data transmission line and for converting bipolar digital data received on said digital data transmission line to digital data suitable for transmission on said data bus.
 26. A maintenance circuit for a digital multiplexed communication system having a central office terminal and a remote terminal with a plurality of digital-to-analog and analog-to-digital line card interface circuits for individual telephone lines, said maintenance circuit located in said central office terminal to automatically, continuously test said line card interface circuits whenever they are idle, comprising:a data bus coupled to said line card interface circuits for carrying digital data; a generator means coupled to said data bus for generating a plurality of words of digital test data; a detector means coupled to said data bus for detecting test data and for generating an output signal; and means to automatically, continuously determine which said line card interface circuits are idle and for causing said generator means to send digital test data to said idle line card interface circuits and for causing said detector means to be coupled to said idle line card interface circuits to detect the presence of said digital test data after it has been converted to analog form and then back to digital form by said idle line card interface circuits.
 27. A self-testing digital multiplexed telephone system for coupling a plurality of telephone lines from a central office switching network to a plurality of remotely located telephone lines, comprising:first means for converting analog signals from said central office telephone lines to digital data for transmission to said remote telephone lines and for converting incoming digital data from said remote telephone lines into analog data for application to said telephone lines from said central office, and including audio circuitry for driving the analog signals in both directions; second means for carrying digital data between said first means and said remote telephone lines; third means for multiplexing digital data from said first means onto said second means and for demultiplexing data from said second means for coupling by said first means to said telephone lines from said central terminal; and fourth means for continuously selecting idle telephone line channels in said first means and for applying a digital test signal to said idle channels through a self-test time slot in said third means, and for detecting a digital test signal after it has passed through said third means and said first means after being converted to an analog test signal, passed through said audio driver circuitry of said first means and reconverted to digital test signal.
 28. An apparatus as defined in claim 27, further comprising:fifth means for converting digital data from said first means to analog data and applying it to said remote telephone lines through a line driver audio amplifier and for receiving analog data from said telephone lines through a line receiver audio amplifier and for converting it to digital data for transmission to said first means; sixth means for multiplexing digital data from said fifth means onto said second means and for demultiplexing digital data from said second means for conversion to analog signals and coupling by said fifth means to said remote telephone lines; and seventh means for continuously selecting idle telephone line channels in said fifth means and for applying a digital test signal to said idle channels through a self-test time slot in said sixth means, and for detecting a digital test signal after it has passed through said sixth means and after, in said fifth means, said digital test signal has been converted to an analog test signal, propagated through all audio amplifiers interfacing with the idle channel, and reconverted to a digital signal.
 29. A self-testing multiplexed telephone system for carrying signals between a plurality of telephone lines from a central office switching network and a plurality of remote telephone lines, comprising:a first means including audio line driver and receiver amplifiers for converting analog signals from said telephone lines from said central office switching network to digital data and for converting digital data to analog signals to drive said central office telephone lines; a second means for carrying digital data; a third means including audio line driver and receiver amplifiers to convert digital data from said first means to analog signals to drive said remote telephone lines and to convert analog signals from said remote telephone lines to digital data; a fourth means for multiplexing said digital data from said first means onto said second means and for demultiplexing data from said second means for use by said first means in driving said central office telephone lines; a fifth means for demultiplexing data from said second means for use by said third means to drive said remote telephone lines and for multiplexing digital data from said third means onto said second means; a sixth means for continuously selecting telephone lines from said central office which are not in use and for causing a test signal to be propagated through all the audio and conversion circuits of said first means interfacing with the idle lines only and for detecting said test signal after it has propagated through said first means to determine the operability of the interface circuits of said idle lines.
 30. An apparatus as defined in claim 29, further comprising:seventh means for continuously selecting remote telephone lines which are not in use and for causing a test signal to be propagated through all the audio amplifiers and conversion circuits of said third means interfacing with the idle lines only and for detecting said test signal after it has propagated through said third means to determine the operability of the interface circuits of said idle lines.
 31. An apparatus as defined in claim 1 or 2 or 26 or 27 or 28 or 29 or 30, further comprising:means for generating a test signal representing a fixed frequency analog signal; and means for determining whether the test signal returned from the telephone line card interface circuitry has a predetermined frequency.
 32. A method of self-testing a terminal in a digital multiplexed telephone system having a multiplexer and a plurality of line interface circuits each including an audio line driver and line receiver which couple said terminal to a plurality of telephone lines, comprising the steps of:(a) determining which of said telephone lines is idle; (b) propagating a test signal through said multiplexer and line interface circuits, including the audio amplifier line drivers and receivers coupled to said idle telephone lines; (c) detecting the amplitude of said test signal after it has passed through the multiplexer and line interface circuits corresponding to the idle telephone lines.
 33. The method of claim 32 wherein the step of detecting the test signal includes the step of determining the frequency of the test signal after it has passed through the multiplexer and line interface circuits corresponding to said idle telephone lines.
 34. The method of claim 32 or 33, further comprising the steps (a), (b), and (c) performed independently at both the remote terminal and the central office terminal ends of said telephone system. 